Gerrit controller for tide, implement provider interface for Gerrit

[chaodaiG]

This PR implements the provider interface, but not hooked up yet. Intentionally left mergePRs method unimplemented as I would expect it to start with dry-run for a bit. Foreseeable remaining works:

• Hook up inrepoconfig for Gerrit.
• Parallelize query.
• Figuring out how to report back to Gerrit. These will be addressed in following PRs.

/cc @cjwagner @mpherman2 @listx cc @alvaroaleman

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chaodaiG

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[chaodaiG]

Will add unit tests once get a first round of agreement on the approach from this PR

[chaodaiG]

Unit tests added, please review @cjwagner @mpherman2 @listx

[listx]

/lgtm

[chaodaiG]

Sorry this took so long. Pretty much all nits. Feel free to ignore. Thank you a lot! Please re-add lgtm

[chaodaiG]

/unhold