Add a py.typed file for submodule

[crwilcox]

To be in accordance with PEP 561 we should add a py.typed file to google/cloud/bigtable/py.typed

The file can be empty but does need to be present for some type checkers.

[crwilcox]

Reopening as to be done, we ought to ensure passage. Do note, you may need to run mypy, pyright, pytype against our samples before merging. It will ensure that we aren't exposing users to failing type checks.

Ideally we would add this to CI as well, to avoid future breakage.

[crwilcox]

TODO: #448 is merged, but currently the code as merged is validating the package, google.cloud.bigtable.

We want to evaluate mypy against google and tests

Once https://www.github.com/googleapis/gapic-generator-python/issues/1026 is addressed, we should be unblocked to wrap this up.

[asottile-sentry]

the py.typed file was added to this package, but the functions aren't actually typed leading to lots of errors of the type:

error: Call to untyped function "Client" in typed context  [no-untyped-call]