feat(coderd): refactors github pr sync functionality (#22715)

- Adds `_API_BASE_URL` to `CODER_EXTERNAL_AUTH_CONFIG_`
- Extracts and refactors existing GitHub PR sync logic to new packages
`coderd/gitsync` and `coderd/externalauth/gitprovider`
- Associated wiring and tests

Created using Opus 4.6

coderd/apidoc/docs.go

@@ -15269,6 +15269,10 @@ const docTemplate = `{
         "codersdk.ExternalAuthConfig": {
             "type": "object",
             "properties": {
+                "api_base_url": {
+                    "description": "APIBaseURL is the base URL for provider REST API calls\n(e.g., \"https://api.github.com\" for GitHub). Derived from\ndefaults when not explicitly configured.",
+                    "type": "string"
+                },
                 "app_install_url": {
                     "type": "string"
                 },

coderd/apidoc/swagger.json

@@ -13792,6 +13792,10 @@
 		"codersdk.ExternalAuthConfig": {
 			"type": "object",
 			"properties": {
+				"api_base_url": {
+					"description": "APIBaseURL is the base URL for provider REST API calls\n(e.g., \"https://api.github.com\" for GitHub). Derived from\ndefaults when not explicitly configured.",
+					"type": "string"
+				},
 				"app_install_url": {
 					"type": "string"
 				},