Add multipart response parsing to HTTPX

Normalized profile

What this task asks for

Add Response iterators that parse multipart HTTP response bodies into parts. Add synchronous and asynchronous multipart iterators that yield `MultipartPart` values from `multipart/*` responses. Parse and validate the final boundary parameter case-insensitively, rejecting malformed media types and boundary values. Handle preambles, epilogues, exact delimiters, and LF, CRLF, or CR line endings across chunks. Preserve duplicate and continued headers while rejecting malformed part headers. Exclude delimiter line endings from part bodies and allow a closing boundary to yield zero parts. Consume and close streaming responses once while keeping in-memory multipart iteration repeatable.