feat: aiohttp client support in HttpPlugin

[elijahr]

Summary

Extend HttpPlugin to intercept aiohttp.ClientSession requests in addition to the existing httpx, requests, and urllib transports.

Motivation

aiohttp is one of the most popular async HTTP clients in the Python ecosystem. The README currently notes that aiohttp is not supported. Adding it closes the last major gap in HTTP client coverage.

Proposed Design

• Add aiohttp transport interception alongside existing httpx/requests/urllib
• Same interaction recording format (method, url, request_headers, request_body, status, response_headers, response_body)
• Same assert_request() / assert_response() chaining pattern
• Optional dependency: bigfoot[aiohttp] extra
• Could be implemented as an enhancement to existing HttpPlugin rather than a separate plugin

Acceptance Criteria

• aiohttp.ClientSession.request (and shorthand methods) intercepted
• Same 7-field interaction model as existing HTTP transports
• bigfoot[aiohttp] optional extra
• Guide updated with aiohttp examples
• Tests covering async aiohttp usage