Scan diffs for changes commits via UI

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/491282

**Problem**

Currently, diff scan is applied only to git pushes via ssh/https
protocol, because there was no way to distiguishing between RPC actions
that triggered the scan.

For example, we want to skip the check for merge commit creation but
verify commits created via WebIDE.

https://gitlab.com/gitlab-org/gitlab/-/issues/423992 introduces an
option to pass additional context from Gitaly RPC endpoint to the
checker code.

**Solution**

* Provide `enable_secrets_check` to selected RPC endpoints
* Run diff scan for checks triggered by calls to these endpoints