feat(web): Support TLS encryption (#322)

* Basic setup to serve HTTPS

* Correctly handle the case of missing TLS configs

* Documenting TLS

* Refactor TLS configuration setup

* Add TLS Encryption section again to README

* Extending TOC in README

* Moving TLS settings to subsection of web settings

* Adding tests for config/web

* Add test for handling TLS

* Rename some variables as suggested

* Corrected error formatting

* Update test module import

* Polishing the readme file

* Error handling for TLSConfig()

---------

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

controller/controller_test.go

@@ -10,6 +10,7 @@ import (
 	"github.com/TwiN/gatus/v5/config"
 	"github.com/TwiN/gatus/v5/config/web"
 	"github.com/TwiN/gatus/v5/core"
+	"github.com/TwiN/gatus/v5/test"
 )
 
 func TestHandle(t *testing.T) {
@@ -45,6 +46,41 @@ func TestHandle(t *testing.T) {
 	}
 }
 
+func TestHandleTls(t *testing.T) {
+	privateKeyPath, publicKeyPath := test.UnsafeSelfSignedCertificates(t.TempDir())
+	cfg := &config.Config{
+		Web: &web.Config{
+			Address: "0.0.0.0",
+			Port:    rand.Intn(65534),
+			Tls:     (web.TLSConfig{CertificateFile: publicKeyPath, PrivateKeyFile: privateKeyPath}),
+		},
+		Endpoints: []*core.Endpoint{
+			{
+				Name:  "frontend",
+				Group: "core",
+			},
+			{
+				Name:  "backend",
+				Group: "core",
+			},
+		},
+	}
+	_ = os.Setenv("ROUTER_TEST", "true")
+	_ = os.Setenv("ENVIRONMENT", "dev")
+	defer os.Clearenv()
+	Handle(cfg)
+	defer Shutdown()
+	request, _ := http.NewRequest("GET", "/health", http.NoBody)
+	responseRecorder := httptest.NewRecorder()
+	server.Handler.ServeHTTP(responseRecorder, request)
+	if responseRecorder.Code != http.StatusOK {
+		t.Error("expected GET /health to return status code 200")
+	}
+	if server == nil {
+		t.Fatal("server should've been set (but because we set ROUTER_TEST, it shouldn't have been started)")
+	}
+}
+
 func TestShutdown(t *testing.T) {
 	// Pretend that we called controller.Handle(), which initializes the server variable
 	server = &http.Server{}