feat(alerting): Add GitLab alerting provider (#485)

* feat: ✨ add gitlab provider

* docs: 📝 add documentation for gitlab provider

* docs: 📝 fix removed github screenshot

* refactor: ➖ remove uniuri dependency

* refactor: 🎨 correctly capitalize GitLab

* Update alerting/alert/type.go

* fix: 💡 correct comments about webhook url and authorization key

* Update alerting/provider/gitlab/gitlab.go

* Update alerting/provider/gitlab/gitlab_test.go

---------

Co-authored-by: TwiN <twin@linux.com>

README.md

@@ -51,6 +51,7 @@ Have any feedback or questions? [Create a discussion](https://github.com/TwiN/ga
     - [Configuring Discord alerts](#configuring-discord-alerts)
     - [Configuring Email alerts](#configuring-email-alerts)
     - [Configuring GitHub alerts](#configuring-github-alerts)
+    - [Configuring GitLab alerts](#configuring-gitlab-alerts)
     - [Configuring Google Chat alerts](#configuring-google-chat-alerts)
     - [Configuring Matrix alerts](#configuring-matrix-alerts)
     - [Configuring Mattermost alerts](#configuring-mattermost-alerts)
@@ -411,6 +412,7 @@ ignored.
 | `alerting.discord`     | Configuration for alerts of type `discord`. <br />See [Configuring Discord alerts](#configuring-discord-alerts).             | `{}`    |
 | `alerting.email`       | Configuration for alerts of type `email`. <br />See [Configuring Email alerts](#configuring-email-alerts).                   | `{}`    |
 | `alerting.github`      | Configuration for alerts of type `github`. <br />See [Configuring GitHub alerts](#configuring-github-alerts).                | `{}`    |
+| `alerting.gitlab`      | Configuration for alerts of type `gitlab`. <br />See [Configuring GitLab alerts](#configuring-gitlab-alerts).                | `{}`    |
 | `alerting.googlechat`  | Configuration for alerts of type `googlechat`. <br />See [Configuring Google Chat alerts](#configuring-google-chat-alerts).  | `{}`    |
 | `alerting.matrix`      | Configuration for alerts of type `matrix`. <br />See [Configuring Matrix alerts](#configuring-matrix-alerts).                | `{}`    |
 | `alerting.mattermost`  | Configuration for alerts of type `mattermost`. <br />See [Configuring Mattermost alerts](#configuring-mattermost-alerts).    | `{}`    |
@@ -550,6 +552,47 @@ endpoints:
 
 ![GitHub alert](.github/assets/github-alerts.png)
 
+#### Configuring GitLab alerts
+| Parameter                           | Description                                                                                                | Default       |
+|:------------------------------------|:----------------------------------------------------------------------------------------------------------------|:--------------|
+| `alerting.gitlab`                   | Configuration for alerts of type `gitlab`                                                                       | `{}`          |
+| `alerting.gitlab.webhook-url`       | GitLab alert webhook URL (e.g. `https://gitlab.com/hlidotbe/example/alerts/notify/gatus/xxxxxxxxxxxxxxxx.json`) | Required `""` |
+| `alerting.gitlab.authorization-key` | Personal access token to use for authentication. <br />Must have at least RW on issues and RO on metadata.      | Required `""` |
+| `alerting.gitlab.severity`          | Override default severity (critical), can be one of `critical, high, medium, low, info, unknown`                | `""`          |
+| `alerting.gitlab.monitoring-tool`   | Override the monitoring tool name (gatus)                                                                       | `"gatus"`     |
+| `alerting.gitlab.environment-name`  | Set gitlab environment's name. Required to display alerts on a dashboard.                                       | `""`          |
+| `alerting.gitlab.service`           | Override endpoint displayname                                                                                   | `""` |
+| `alerting.gitlab.default-alert`     | Default alert configuration. <br />See [Setting a default alert](#setting-a-default-alert).                     | N/A           |
+
+The GitLab alerting provider creates an alert prefixed with `🚨 (gatus):` and suffixed with the endpoint's display
+name for each alert. If `send-on-resolved` is set to `true` on the endpoint alert, the alert will be automatically
+closed when the alert is resolved. See
+https://docs.gitlab.com/ee/operations/incident_management/integrations.html#configuration to configure the endpoint.
+
+```yaml
+alerting:
+  gitlab:
+    webhook-url: "https://gitlab.com/hlidotbe/example/alerts/notify/gatus/xxxxxxxxxxxxxxxx.json"
+    authorization-key: "12345"
+
+endpoints:
+  - name: example
+    url: "https://twin.sh/health"
+    interval: 5m
+    conditions:
+      - "[STATUS] == 200"
+      - "[BODY].status == UP"
+      - "[RESPONSE_TIME] < 75"
+    alerts:
+      - type: gitlab
+        failure-threshold: 2
+        success-threshold: 3
+        send-on-resolved: true
+        description: "Everything's burning AAAAAHHHHHHHHHHHHHHH"
+```
+
+![GitLab alert](.github/assets/gitlab-alerts.png)
+
 
 #### Configuring Google Chat alerts
 | Parameter                                     | Description                                                                                 | Default       |