Add support for constraints.txt files

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.

Start by tracing rules_uv's implementation of `uv pip compile` and how its `args` and `extra_args` are passed to the rule. Review pip's constraints-file behavior and determine whether direct constraints-file support or location references is the intended approach. Done means users can supply a constraints.txt file when compiling dependencies.

Written by the indexing model from the issue text.