add helpers for Flask's send_file wrapper

[davidism]

Without these changes, Flask's wrappers would need to duplicate the path processing code. These changes are for internal use only.

• _root_path private parameter allows paths to be relative to app.root_path instead of cwd.
• max_age can be a callable that takes an optional path, allows calling app.get_send_file_max_age.

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.