gh-69152: add method get_proxy_response_headers to HTTPConnection class (#104248)

Add http.client.HTTPConnection method get_proxy_response_headers() - this is a followup to https://github.com/python/cpython/pull/26152 which added it as a non-public attribute. This way we don't pre-compute a headers dictionary that most users will never access. The new method is properly public and documented and triggers full proxy header parsing into a dict only when actually called.

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Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>