[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.

Look at the `a2aclient` package to understand the existing `WithConnectionPool()` factory option and how connections are currently managed. Examine the linked ADK RemoteAgent integration to see the constructor pattern. Determine what a connection pool interface needs to provide and where to integrate it into the client lifecycle. A successful implementation will allow the client to reuse connections across invocations.

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