[Python] BigQuery handler for enrichment transform

[riteshghorse]

Implemented the BigQuery handler for Enrichment transform.

To use this handler you need either of the following combinations:

1. table_name, row_restriction_template, fields
2. table_name, row_restriction_template, condition_value_fn
3. query_fn

In addition to this, batch size can be specified with min_batch_size and max_batch_size. These are passed to the BatchElements transform just before making calls to BigQuery. This helps reducing the cost on the BigQuery side.

By default, the handler pulls all columns from the BigQuery table. To override this, use the column_name parameter to specify a list of column names to fetch.

The user interface here is pretty similar to TableReadOptions.

Example Usage:

handler = BigQueryEnrichmentHandler(
        project=PROJECT_ID,
        row_restriction_template="id='{}'",
        table_name=TABLE_NAME,
        fields=['id'],
        min_batch_size=2,
        max_batch_size=100,
    )

---

Thank you for your contribution! Follow this checklist to help us incorporate your contribution quickly and easily:

• Mention the appropriate issue in your description (for example: addresses #123), if applicable. This will automatically add a link to the pull request in the issue. If you would like the issue to automatically close on merging the pull request, comment fixes #<ISSUE NUMBER> instead.
• Update CHANGES.md with noteworthy changes.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

See the Contributor Guide for more tips on how to make review process smoother.

To check the build health, please visit https://github.com/apache/beam/blob/master/.test-infra/BUILD_STATUS.md

GitHub Actions Tests Status (on master branch)

See CI.md for more information about GitHub Actions CI or the workflows README to see a list of phrases to trigger workflows.

[riteshghorse]

R: @damccorm

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[damccorm]

Overall looks good, had some minor comments

[riteshghorse]

Thanks for the review! Made the suggested changes.

[damccorm]

LGTM once checks pass, thanks!

[riteshghorse]

BigQuery tests passed in 3.9-

Known failure in PostCommit 3.8 due to dependency conflicts