Enrichment Transform with BigTable handler

[riteshghorse]

This PR contains the implementation of:

1. Enrichment transform and its tests.
2. BigTable handler for enrichment and integration test.
3. Default Throttler and Exponential Backoff of requests.

Cross-lang enablement and caching will be added in subsequent PRs.

Part of #29787

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[damccorm]

Mostly looks good, though I didn't get a chance to look at the enrichment_handlers folder yet. Leaving comments for the pieces I've reviewed so far

[riteshghorse]

Thanks for the review! Made the suggested changes.

[damccorm]

BigTable piece mostly LGTM as well, just have some lingering comments

[riteshghorse]

Thanks! Made the changes from last review. Throttle test cases are left. Feel free to defer the review until that or up to you!

[riteshghorse]

Added the throttle unit test. Bigtable improvements are remaining now.

[damccorm]

LGTM - thanks!

[riteshghorse]

Thank you for your reviews! I'll merge once the tests pass

[riteshghorse]

Tests passed. Removing the trigger file