Support for KafkaToPubsub template to retrieve Kafka credentials from GCS

[KhaninArtur]

This PR consists of the following improvements for the KafkaToPubsub template:

• Added support to retrieve Kafka credentials form GCS
• Added SSL certificates validation
• Added support for different Kafka SASL mechanisms

[google-cla[bot]]

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[ilya-kozyrev]

@googlebot I consent.

[google-cla[bot]]

All (the pull request submitter and all commit authors) CLAs are signed, but one or more commits were authored or co-authored by someone other than the pull request submitter.

We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

Note to project maintainer: There may be cases where the author cannot leave a comment, or the comment is not properly detected as consent. In those cases, you can manually confirm consent of the commit author(s), and set the cla label to yes (if enabled on your project).

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[RaphaelSanamyan]

@googlebot I consent.

[KhaninArtur]

Hi @prathapreddy123, did you have a chance to go through this PR? Is there something we can help you with?

[prathapreddy123]

Hi @prathapreddy123, did you have a chance to go through this PR? Is there something we can help you with?

Hi Artur, I will complete review by mid next week

[stale[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[bvolpato]

Closing this because it got stale for a few months. Please rebase / file a new one to bring it up to the top of the stack.