Upload API should accept signature files as separate requests

[di]

What's the problem this feature will solve?
Currently, signatures are required to be uploaded as part of the POST request for a given distribution file, rather than as separate files:

warehouse/warehouse/forklift/legacy.py

Lines 1314 to 1325 in 3ceafd7

	# Also buffer the entire signature file to disk.
	if "gpg_signature" in request.POST:
	has_signature = True
	with open(os.path.join(tmpdir, filename + ".asc"), "wb") as fp:
	signature_size = 0
	for chunk in iter(
	lambda: request.POST["gpg_signature"].file.read(8096), b""
	):
	signature_size += len(chunk)
	if signature_size > MAX_SIGSIZE:
	raise _exc_with_message(HTTPBadRequest, "Signature too large.")
	fp.write(chunk)

This leads to confusion when users are attempting to upload distribution files and signatures as separate invocations, such as pypa/twine#931.

Describe the solution you'd like
The upload endpoint should accept signature files as separate requests, rather than as part of a POST request for a distribution file that it corresponds to.

Additional context
An 'upload 2.0' API is described in https://peps.python.org/pep-0694/, but has the same issue.