Replace Source type for BigTable and Kafka stress tests

[akashorabek]

Replaced PeriodicImpulse source to SyntheticUnboundedSource for BigTable and Kafka stress tests since there is a difference in throughput and effiency.

---

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.

[github-actions]

Assigning reviewers. If you would like to opt out of this review, comment assign to next reviewer:

R: @johnjcasey added as fallback since no labels match configuration

Available commands:

• stop reviewer notifications - opt out of the automated review tooling
• remind me after tests pass - tag the comment author after tests pass
• waiting on author - shift the attention set back to the author (any comment or push by the author will return the attention set to the reviewers)

The PR bot will only process comments in the main thread (not review comments).

[Abacn]

Another PR used strimzi Kafka server #30967 found not working with apache-beam-testing project (cc: @volatilemolotov). Are the kafka k8s change tested, or could you please share a Kafka ST test result on this PR?

Bigtable changes LGTM

[akashorabek]

Yes, kafka k8s changes were tested. Here are the links to tests that use this changes:

Write kafka test
Read kafka test

[Abacn]

Thank you!