Missing PEP561 marker file for google.cloud.bigtable

[axyjo]

It looks like #920 accidentally removed this file.

[honnix]

Could it be the deletion is on purpose, as the lib is generally not well typed? googleapis/google-cloud-python#15334 (comment)

[augi]

I can confirm that we have our builds broken on error: Call to untyped function "Client" in typed context [no-untyped-call] after updating from 2.30.1 to 2.31.0.
Was that change really desired? If so, what is please the migration plan? Thanks!