Add GitGuardian integration

Overview

This issue is the first step to implementing GitGuardian secrets detection: GitGuardian pre-receive secrets detection (&11494 - closed).

Within this issue, a customer should be able to add an integration and specify GitGuardian API token:

Proposal

A customer goes to Settings > Integrations of a project, group, or instance and chooses to add a new GitGuardian integration. That integration will later be used to create an HTTP call to https://api.gitguardian.com/docs#operation/content_scan to check whether a file violates any policy regarding secrets.

Developer documentation for introducing a new integration: https://docs.gitlab.com/ee/development/integrations/

Very rough draft MR: Draft: Git guardian (!140098 - closed)

Additional requirement

• Let's put it behind a feature flag in order to keep it disabled until the functionality is fully implemented
• Let's hide this integration for Gitlab.com users and have it visible for Self-managed/Dedicated instances