Fix the memory issue in WSGI middleware

[peterzaccha]

Summary

This PR attempts to fix the WSGIMiddleware, which has a serious memory consumption issue when handling large request and response sizes by:

• Limiting memory usage by reducing the max buffer size in create_memory_object_stream from math.inf to zero.
• Introducing WsgiRequestBody class for incremental handling of the request body, replacing the previous byte array loading approach.

The implementation for WSGIMiddleware was

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[Kludex]

The middleware is deprecated. Use the a2wsgi package instead.