Accepts `PathLike` objects for `model_dir` (#16200)

* Accepts `PathLike` objects for `model_dir`

* Retrieves the file system path representation if `PathLike` object
is passed to `Estimator` or `RunConfig` for `model_dir`, instead of
`str`.
* Closes #15784

* Renames `as_path_repr` to `path_to_str`
* Adds `path_to_str` to "compat.py"'s top level docstring

* Updates golden on addition of a new method
* commit 5242790 adds method `path_to_str` in "compat.py"