Add type annotations to signatures

[tswast]

@alimcmaster1 added type signatures to the gbq wrapper in pandas in pandas-dev/pandas#30632 These types can also be useful for folks that use pandas-gbq directly.

• Add type annotations to API
• Add type annotations to code samples
• Add test session(s) to check types
• Publish py.typed file

[tswast]

When we do this, we may want to install the now officially supported pandas stubs to ensure pandas-gbq type checks.