auth: AuthorizedHttp.configure_mtls_channel() drops custom PoolManager settings

先在 packages/google-auth/google/auth/transport/urllib3.py 中阅读 _make_mutual_tls_http() 和 AuthorizedHttp.configure_mtls_channel()，然后比较 requests 传输中的保留行为。更新 packages/google-auth/tests/transport/test_urllib3.py 中相关的单元测试覆盖范围，确保自定义的重试、池大小、超时和标头设置在 mTLS 配置后仍然保留，并运行这些测试。

In packages/google-auth/google/auth/transport/urllib3.py, calling AuthorizedHttp.configure_mtls_channel() replaces self.http by calling _make_mutual_tls_http(cert, key), which constructs a new urllib3.PoolManager(ssl_context=ctx) without copying non-SSL pool settings from the existing self.http instance.

When callers initialize AuthorizedHttp(credentials, http=custom_pool_manager) with custom retry policies (retries), connection pool sizing (maxsize, num_pools), timeouts (timeout), or default headers (headers), calling configure_mtls_channel() discards all of those custom configurations and emits a UserWarning stating that the http object provided in the constructor is overwritten.

By contrast, AuthorizedSession.configure_mtls_channel() in packages/google-auth/google/auth/transport/requests.py preserves custom retry and connection pool sizing (max_retries, pool_connections, pool_maxsize, pool_block) from the existing HTTPAdapter when mounting _MutualTlsAdapter.

Proposed Fix

Update _make_mutual_tls_http() in packages/google-auth/google/auth/transport/urllib3.py to accept optional PoolManager keyword arguments, and update AuthorizedHttp.configure_mtls_channel() to extract non-SSL configuration settings (retries, maxsize, block, timeout, headers, and num_pools) from self.http and forward them when constructing the mTLS PoolManager.

Update unit tests in packages/google-auth/tests/transport/test_urllib3.py to verify that custom retry, pool sizing, timeout, and header configurations on a user-provided PoolManager are preserved across configure_mtls_channel() calls.