feat: reloadable tls client config

[shuiyisong]

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What's changed and what's your intention?

This patch introduces the ability for channel_manager to use a re-loadable client TLS config to automatically reload the TLS files when changed.
The ReloadableTlsServerConfig is now extracted into reloadable_tls for reuse.

PR Checklist

Please convert it to a draft if some of the following conditions are not met.

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.
• API changes are backward compatible.
• Schema or data changes are backward compatible.

[sunng87]

To be clear, in this patch we only changed intra-cluster grpc client to reload certificate. The grpc server is not covered, right?

[shuiyisong]

To be clear, in this patch we only changed intra-cluster grpc client to reload certificate. The grpc server is not covered, right?

Yes, this only change the gRPC client(channel manager) to be able to reload the TLS change. The gRPC server of the database is not changed.

[sunng87]

This can introduce an issue if all clients reloads certs automatically but server doesn't. The updated client will not be able to connect to server without a restart.

[sunng87]

Confirmed with @shuiyisong , this will be used for communication between database and external services like authentication providers or something.

But we still need to ensure the cert reload will work if intra-cluster grpc is configured with tls.