Add support for constraints.txt files

[nate-selzer]

constraints.txt files are used when you need to specify a version constraint for a non-direct dependency. This is also supported by uv. It would be good if rules_uv's implementation of uv pip compile allowed you to easily specify the constraints file.
https://pip.pypa.io/en/stable/user_guide/#constraints-files

A quick fix would also be to allow location references on the args and extra_args arguments of the rule. This would let users pass arbitrary files as arguments to pip-compile.