BEAM-13261 added max connections setting

[pas725]

Adds max pool connections as a configurable parameter while creating Datasource.

Task: #21200

---

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.

[pas725]

R: @jbonofre
R: @timrobertson100

[github-actions]

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

[aromanenko-dev]

Thanks! LGTM
Just a small note, ptal

[Abacn]

This PR broke Python postcommi's jdbc xlang test: https://ci-beam.apache.org/view/PostCommit/job/beam_PostCommit_Python38/3630/

 with ExternalTransform.service(self._expansion_service) as service:
response = service.Expand(request)
if response.error:
> raise RuntimeError(response.error)
E RuntimeError: java.lang.IllegalArgumentException: Cannot find field maxConnections in schema Fields:

[Abacn]

we should tolerate the incoming schema not containing this field for backward compatibility