Spike: Add a push check that performs secret detection

Context

As part of discovery work for #422574 (closed), add a new push check to Rails which gets invoked by Gitaly whenever a user attempts to push commits to a repository.

Proposal

The aim of the spike is to determine:

• what the user interaction looks like -- are we able to define an error message that a user would then be able to see as the output of a failed git push invocation?
• if the outcome of #423832 (closed) suggests we use a Go implementation, the best way of shipping a new Go binary with a GitLab installation and invoking it from Rails.
• how false positives should be handled. Two options are proposed, but using a special flag in the commit message seems like the ideal approach for forcing a push containing secrets to succeed.

Push check classes in Rails seem to be located here.

Additional Considerations

• After we complete these two spikes, we'd like to be able to give a high confidence estimate on when we'd be able to deliver Perform secret detection for highest risk conte... (#422574 - closed)
• We'll look to set up a stage-wide brainstorming session in %16.5 to review/discuss the decisions we made as well as any lessons learned.

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Last updated by this job

• TOPIC User interaction in push checks #423834 (comment 1553607364)
• TOPIC Shipping a Go binary in the monolith, and how to invoke it? #423834 (comment 1553607998)
• TOPIC Handling false positives #423834 (comment 1553609088)
• TOPIC Follow-up - determine performance SLA requirements for adding pre-receive secret detection #423834 (comment 1581254467)

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