[Feature] Add an extension point for client connection pooling

`a2aclient.WithConnectionPool()` factory option can simplify connection reuse for integrations like [ADK RemoteAgent](https://github.com/google/adk-go/blob/main/agent/remoteagent/a2a_agent.go#L35), where constructor takes an AgentCard and opens a connection on invocation.

Connection pooling improves application performance by reusing existing connections instead of creating new ones, which reduces overhead and speeds up response times.