BEAM-13261 added max connections setting

[pas725]

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

Task: #21200

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[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