Display success message if secret push protection found no secrets

Proposal

There was some feedback on internal dogfooding of Secret Push Protection:

Some early feedback on the feature. Once we enabled pre-receive SD, I was expecting to see some kind of log message while I was pushing commits. However, I couldn't see anything and that made me think if the feature is working in the first place. It would be nice if there is a logging message mentioning that no secrets were detected.

When secret push protection finds a secret in the user's commits, we show a message like this:

We should also show a message when no secrets are found. After consideration the following message was chosen:

Secret push protection completed with no findings.

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Implementation plan

Based on this discussion about SPP message display mechanisms, I've updated the implementation plan:

Use PostReceiveService to output a message after the push is successfully received:

if secret_push_protection_ran_successfully?
  response.add_basic_message("Secret push protection completed with no findings.")
end

Pros:

• Simpler implementation, similar to MR creation messages
• Works in the post-receive phase where SPP has already completed
• No need for Redis persistence

Cons:

• Message appears after the push completes (not during pre-receive)

Implementation Steps

Add conditional message in PostReceiveService#execute:

   if secret_push_protection_enabled? && secret_push_protection_completed_successfully?
     response.add_basic_message("Secret push protection completed with no findings.")
   end

Old implementation plan

Add success message to ee/lib/gitlab/checks/secrets_check.rb, display when ::Gitlab::SecretDetection::Status::NOT_FOUND.

• Add console message to ee/lib/ee/gitlab/git_access.rb or perhaps a GitAccessResult::CustomAction with the success message.

Note: Check this comment which has a suggested implementation plan that may work better.