Option to Disable MR Pipeline

Proposal

It would be nice to make the behavior of MR pipelines vs. branch pipelines configurable.

This is already the case for the official scanners like dependency scanning: https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Jobs/Dependency-Scanning.latest.gitlab-ci.yml?ref_type=heads#L19 . There is the variable AST_ENABLE_MR_PIPELINES to define if scans should run in MR pipeline or branch pipeline.

At the moment this component forces us to use merge request pipelines. I do not see any benefit of these, only more complex configuration to distinguish between branch and merge request pipeline. It would be much easier to just always run a branch pipeline.

At least it should be allowed to decide on our own. I would prefer an input over a variable.