feat(http): add keepalive support for fetch requests (angular#60621)

This commit adds support for the Fetch API's keepalive option when using HttpClient with the withFetch provider.

The change includes:
- Added keepalive to HttpRequestInit interface
- Modified FetchBackend to pass the option
- Added some unit test

PR Close angular#60621

Author: SkyZeroZx

goldens/public-api/common/http/errors.api.md

@@ -21,6 +21,8 @@ export const enum RuntimeErrorCode {
     // (undocumented)
     JSONP_WRONG_RESPONSE_TYPE = 2811,
     // (undocumented)
+    KEEPALIVE_NOT_SUPPORTED_WITH_XHR = 2813,
+    // (undocumented)
     MISSING_JSONP_MODULE = -2800,
     // (undocumented)
     NOT_USING_FETCH_BACKEND_IN_SSR = 2801,