feat(alerting): implement Gitea alerting provider (#842)

* feat: implement Gitea alerting provider integration

- Add TypeGitea for the gitea alerting provider
- Introduce a new file for the gitea alerting provider implementation
- Implement the AlertProvider struct with necessary fields for gitea integration
- Add validation logic for the AlertProvider configuration
- Create tests for the AlertProvider's validation and sending functionality
- Update go.mod to include the gitea SDK as a dependency
- Modify the alerting configuration validation to recognize TypeGitea

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

* chore: integrate Gitea alerting provider configuration

- Add Gitea alerting provider import to the configuration file
- Update the comment for the RepositoryURL field to reflect Gitea instead of GitHub

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

* feat: add Assignees support to AlertProvider functionality

- Add a field for Assignees to the AlertProvider struct
- Update the Send function to include Assignees in the alert payload

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

* feat: implement Gitea alerting configuration and documentation

- Add a new image asset for Gitea alerts
- Update the README to include configuration details for Gitea alerts
- Introduce parameters for Gitea alerting, including repository URL and personal access token
- Document the behavior of the Gitea alerting provider regarding issue creation and resolution
- Include an example YAML configuration for Gitea alerts

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

* Update README.md

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

* Update README.md

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

* Update README.md

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

* feat: refactor AlertProvider for improved client configuration

- Add import for the Gatus client library
- Remove the SkipVerify field from the AlertProvider struct
- Introduce ClientConfig field in the AlertProvider struct for client configuration
- Update validation logic to check for ClientConfig instead of SkipVerify

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

* Update README.md

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

* chore: update configuration for Gitea integration

- Change references from GitHub to Gitea in the configuration section
- Update alerting provider descriptions to reflect the correct platform
- Swap the order of GitHub and Gitea configurations
- Replace Gitea alert image with GitHub alert image
- Adjust the type field from gitea to github in the relevant sections

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

* fix: ensure ClientConfig is validated and defaults set

- Add a check for nil ClientConfig in the IsValid function
- Set ClientConfig to a default configuration if it is nil

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

---------

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>
Co-authored-by: TwiN <twin@linux.com>

alerting/provider/provider.go

@@ -6,6 +6,7 @@ import (
 	"github.com/TwiN/gatus/v5/alerting/provider/custom"
 	"github.com/TwiN/gatus/v5/alerting/provider/discord"
 	"github.com/TwiN/gatus/v5/alerting/provider/email"
+	"github.com/TwiN/gatus/v5/alerting/provider/gitea"
 	"github.com/TwiN/gatus/v5/alerting/provider/github"
 	"github.com/TwiN/gatus/v5/alerting/provider/gitlab"
 	"github.com/TwiN/gatus/v5/alerting/provider/googlechat"
@@ -66,6 +67,7 @@ var (
 	_ AlertProvider = (*email.AlertProvider)(nil)
 	_ AlertProvider = (*github.AlertProvider)(nil)
 	_ AlertProvider = (*gitlab.AlertProvider)(nil)
+	_ AlertProvider = (*gitea.AlertProvider)(nil)
 	_ AlertProvider = (*googlechat.AlertProvider)(nil)
 	_ AlertProvider = (*jetbrainsspace.AlertProvider)(nil)
 	_ AlertProvider = (*matrix.AlertProvider)(nil)