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/externalauth/externalauth_internal_test.go

@@ -25,6 +25,7 @@ func TestGitlabDefaults(t *testing.T) {
 			DisplayName:                   "GitLab",
 			DisplayIcon:                   "/icon/gitlab.svg",
 			Regex:                         `^(https?://)?gitlab\.com(/.*)?$`,
+			APIBaseURL:                    "https://gitlab.com/api/v4",
 			Scopes:                        []string{"write_repository"},
 			CodeChallengeMethodsSupported: []string{string(promoauth.PKCEChallengeMethodSha256)},
 		}