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

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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

[damccorm]

@Polber @reeba212 friendly reminder to please take a look at this one when you have a chance

[Polber]

You should be able to rebase on master now and pull in the EnrichmentTransform changes

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Reminder, please take a look at this pr: @tvalentyn @Abacn

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[damccorm]

R: @Polber

@Polber @reeba212 what are next steps here?

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[damccorm]

Failing tests are unrelated to the PR, merging - thanks!