Support zero-copy memoryview uploads as request content

[technillogue]

Summary

Previously content=memoryview(...) fell through encode_content to the generic iterable branch, was iterated element-by-element into integers, and raised TypeError: object of type 'int' has no len().

httpx2:

• encode_content now accepts a C-contiguous memoryview (e.g. over a bytearray, array.array, or an mmap) and yields it as a single chunk via a new BufferStream, normalised to a 1-D unsigned-byte view. A non-contiguous view raises rather than being silently copied.
• BufferStream, unlike ByteStream, is treated as (replayable) streaming content - it is not eagerly read into a bytes object at request construction - so the buffer is never materialised up front.
• memoryview is added to RequestContent/ResponseContent.

httpcore2:

• _send_event uses h11's send_with_data_passthrough for Data events. With Content-Length framing the body is written straight to the network rather than via b"".join(...). Chunked framing and header writes are unchanged (coalesced into a single write).
• Body memoryview chunks with itemsize > 1 are normalised to a flat byte view before h11, since both h11's Content-Length tracking (len()) and the backends' byte-count slicing assume one byte per element.

Net effect: a large buffer (e.g. a memoryview over an mmap) is only faulted into memory as it is written to the socket. A 128 MiB upload allocates ~18 MiB peak instead of a full-body copy.

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.

[codspeed-hq]

Merging this PR will not alter performance

✅ 15 untouched benchmarks
⏩ 7 skipped benchmarks¹

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_{Comparing technillogue:memoryview-content-passthrough (c78ecf3) with main (82b9e2d)}

Footnotes

1. 7 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩