requests: Add chunked response body support.

[pablogventura]

Summary

Follow-up to #1124, which added HTTP/1.1 with Content-Length streaming and
raised a ValueError on Transfer-Encoding: chunked responses. This PR adds
incremental decoding of chunked response bodies so requests can consume the
common case of servers that stream without a Content-Length.

A small ChunkedStream wrapper decodes the chunk framing on the fly. It keeps
the existing behaviour intact:

• .raw.read(n) and .raw.readinto(buf) stream the decoded body without
  buffering everything in request() (works for mip).
• .content stays lazy and materialises the full body on demand.
• BodyStream (Content-Length) is untouched; chunked responses select
  ChunkedStream instead.

Chunk extensions after the size are skipped and trailers are discarded.

Testing

• Unix port: micropython python-ecosys/requests/test_requests.py passes,
  including a replaced test that decodes a chunked response via .content and
  a new test that consumes it via .raw.readinto().
• Hardware: ESP32_GENERIC (MicroPython 1.29 preview) over WiFi against
  http://httpbin.org/stream/2 (decoded via .content) and
  http://httpbin.org/stream-bytes/64 (consumed via .raw.readinto() with a
  16-byte buffer). Both returned the expected bytes.

Trade-offs and Alternatives

• Trailers are discarded; there is no API to expose them.
• Chunk extensions are ignored.
• Case-insensitive header matching is out of scope here (tracked in aiohttp module case-insensitive headers #1126).
• .content still materialises the whole body, consistent with the existing
  contract; readinto() is the incremental path.

Generative AI

I used generative AI tools when creating this PR, but a human has checked the
code and is responsible for the code and the description above.

[dpgeorge]

This is a lot of extra code. Can you find a way to reduce the code size, maybe combine this class with BodyStream?

To test code size, run mpy-cross __init__.py and look at the size of the resulting __init__.mpy file. Making that file smaller is the goal.

[pablogventura]

Thanks for the review. I merged chunked decoding into BodyStream with a chunked flag and inlined the chunk header parsing in readinto(), removing the separate ChunkedStream class (commit 4822856).

__init__.mpy sizes measured with mpy-cross against upstream/master (no chunked support):

Version	__init__.mpy	vs master
master (no chunked)	2929 bytes	-
this PR, separate ChunkedStream	3413 bytes	+484 (+16.5%)
this PR, merged into BodyStream	3239 bytes	+310 (+10.6%)

Merging the two classes saves 174 bytes (-5.1%) vs the separate-class version.

Unix tests pass. ESP32 (ESP32_GENERIC over WiFi) re-tested against httpbin chunked endpoints (.content on /stream/2, readinto on /stream-bytes/64).

Trailers are still discarded and chunk extensions are still ignored. The Content-Length code path is unchanged from master.