Add end-to-end response streaming via streamRequest()

[loks0n]

Add streamRequest(RequestInterface, callable $sink) to the Adapter interface, the Client wrapper, and both transports. It delivers the response body to the sink chunk-by-chunk as it arrives, keeping memory bounded regardless of body size; the returned response carries the status and headers with an empty body.

Both adapters use their native push mechanism in the calling context — no curl_multi, no child coroutine:

• cURL forwards CURLOPT_WRITEFUNCTION to the sink (sendRequest and streamRequest now share a transfer() core).
• Swoole forwards write_func to the sink in the main coroutine (sendRequest/streamRequest share perform()).

sendRequest() is unchanged: still PSR-18, buffered, decodable.

Tests (shared AdapterContract, so proven on curl and Swoole):

• stream a body to a sink, asserting incremental delivery
• 8 MiB request and response payloads round-trip; the streamed response holds peak memory under 2 MiB
• redirects are not followed when streaming The test server now runs with post_max_size=64M/memory_limit=256M so large request payloads are accepted.