feat: support triple grpc heart beat

[Lo1nt]

Motivation:

Enable heart beat for triple so as to archive tcp keep alive.

Notice that when we apply MOSN, there is the mechanism that close inactive connection.

Modification:

Add config that set the keep alive interval.

Result:

Allow config for keep alive interval for triple. Default at 0. not enabled.

Summary by CodeRabbit

• New Features

  • Introduced a new configuration option for gRPC clients to manage keep-alive intervals, enhancing connection management.

• Bug Fixes

  • Normalized formatting of constant string declarations for improved readability.

• Tests

  • Added unit tests to validate the initialization of the new keep-alive interval constant in the transport class.