[FEATURE] Add HTTP client transport support

[mchades]

Describe the feature

Add an HTTP client transport (HttpClientTransport) so the SDK can communicate with ADP servers over HTTP, in addition to the existing stdio transport.

Motivation

The adp-hypervisor is adding HTTP transport support (see adp-hypervisor#89). The SDK needs a corresponding client-side HTTP transport to connect to ADP servers running in HTTP mode, without managing a subprocess.

Describe the solution

• Add httpx as a required dependency (following MCP SDK precedent).
• Implement HttpClientTransport that sends JSON-RPC requests via POST and reads the response body.
• Add an http_client() async context manager as a convenience function (mirroring stdio_client()).
• Update README.md with HTTP transport documentation and usage examples.

Additional context

• Related: adp-hypervisor#89 (server-side HTTP transport)
• The HTTP transport uses request-response semantics: each write_message performs the POST and buffers the response; read_message returns it. Concurrency is serialized by ClientSession._send_lock, consistent with the stdio transport approach.