[yaml] Add use cases for Enrichment transform in YAML

[reeba212]

1. Added a simple bigtable enrichment example
2. Added an end to end use case that demonstrates how spanner IO and enrichment transform coupled with other YAML transforms can be used in a real world setting

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R: @ahmedabu98 for label io.

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[Polber]

Thanks! Added a few comments

[Polber]

Is this necessary? It is defined in all the transforms, so not sure why it would be required

[Polber]

Just a couple small changes, thanks!

[github-actions]

Reminder, please take a look at this pr: @damccorm @ahmedabu98

[github-actions]

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[github-actions]

Reminder, please take a look at this pr: @liferoad @johnjcasey

[github-actions]

Assigning new set of reviewers because Pr has gone too long without review. If you would like to opt out of this review, comment assign to next reviewer:

R: @tvalentyn for label python.
R: @Abacn for label io.

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[damccorm]

@Polber what are next steps on this PR?

@reeba212 it looks like some precommit checks are failing - would you mind taking a look? At a glance, I see:

1. the RAT license check is missing - this is because some files are missing the Apache license
2. Lint/format checks are failing - could you try https://cwiki.apache.org/confluence/display/BEAM/Python+Tips#PythonTips-LintandFormattingChecks
3. test_bigtable_enrichment_yaml seems to be failing