Conditionally sign web based commits on Saas

What does this MR do and why?

Background:

• We want to apply settings that would allow groups and projects to opt-in to web-based commits being signed automatically.
• In self-managed instances users can opt-in the entire instance through configuration: https://docs.gitlab.com/user/project/repository/signed_commits/web_commits/
• For saas we want allow groups and projects to opt-in rather than the entire instance

This change is meant to apply the web_based_commit_signing_enabled setting for Saas only. This setting is false by default.

References

• Add controls to allow groups to opt in to web-b... (&17775)
• Call UserCommitFiles with the sign field condit... (#540918 - closed)

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