Adding type hints so static analyzers like mypy can work with google.cloud.storage

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**Is your feature request related to a problem? Please describe.**
Tools like mypy can use type hints to run static analysis on your code to find bugs in incorrect type usage.
**Describe the solution you'd like**
Mypy requires type hints to be exported by the library, either as part of the library and exported per [PEP 484](https://www.python.org/dev/peps/pep-0484/), or as a separate stubs library per [PEP 561](https://www.python.org/dev/peps/pep-0561/).
**Describe alternatives you've considered**
Currently we're ignoring all typing information for this library.
**Additional context**