feat: add support for AddToCell in Data Client

[axyjo]

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Fixes #1146 🦕

[daniel-sanche]

Thanks for putting this together! Do you think you could also add a system test like this, to make sure it works as we expect end-to-end?

Edit: Actually, I ended up putting this together myself. You can copy it in: axyjo/python-bigtable@axyjo-data-client-add-to-cell...googleapis:python-bigtable:data-client-add-to-cell-system-tests

[daniel-sanche]

In case you miss my edit: I put together a system test for this. It would be great if you could merge that in here

[axyjo]

Thanks, @daniel-sanche + @mutianf! Just addressed your feedback + cherry-picked Daniel's changes.

[daniel-sanche]

One more small change, then it looks good!

[daniel-sanche]

LGTM (after checks pass). Thanks for putting this together!

[daniel-sanche]

Sorry, one more change: It looks like some tests are breaking because of the new column family in the system tests. Can you apply this diff to fix the CI? (It also adds another unit test as well)

[axyjo]

@daniel-sanche apologies for the delay -- just pushed up those changes :)

[axyjo]

@daniel-sanche I think these Kokoro failures are unrelated to the changes, right?

[daniel-sanche]

Yes, we can ignore those sample failures

Just merged, thanks for all your work on this!

[axyjo]

Thanks for your help w/ the system tests!