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

codersdk/deployment.go

@@ -980,6 +980,10 @@ type ExternalAuthConfig struct {
 	// 'Username for "https://github.com":'
 	// And sending it to the Coder server to match against the Regex.
 	Regex string `json:"regex" yaml:"regex"`
+	// APIBaseURL is the base URL for provider REST API calls
+	// (e.g., "https://api.github.com" for GitHub). Derived from
+	// defaults when not explicitly configured.
+	APIBaseURL string `json:"api_base_url" yaml:"api_base_url"`
 	// DisplayName is shown in the UI to identify the auth config.
 	DisplayName string `json:"display_name" yaml:"display_name"`
 	// DisplayIcon is a URL to an icon to display in the UI.