[Python] Allow lambda function in bigtable handler to build a custom row key

[riteshghorse]

Allow users to use a custom function to build a composite row key from for the Bigtable row.

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[damccorm]

Overall LGTM, thanks - just had a minor nit. Looks like lint is red too (rest are flakes I think)

[damccorm]

LGTM - added one suggestion, feel free to merge once accepted or comment if you disagree

[riteshghorse]

Unrelated failure in PyCoverage - Task :sdks:python:test-suites:tox:py38:testPy38tft-113 FAILED.

Merging this. Thanks for the review!