Allow integrating release-it into the common pipeline

The release-it template currently needs to be explicitly added to CI pipelines. It would be nice if this was made part of the common pipeline so that other common pipeline jobs can rely on it.

I don't think there's anything blocking that except:

• Satisfy the rule that common pipeline jobs should only trigger if they have reason to believe they are needed by adding an exits:paths:[.release-it.json] rule to the release it jobs.
• Add a RELEASE_IT_DISABLED variable which can be set to disable the jobs if incorrectly triggered.
• Rename the USE_MERGE_REQUEST_RELEASE_FLOW variable to RELEASE_IT_USE_MERGE_REQUEST_RELEASE_FLOW (making the new variable default to $USE_MERGE_REQUEST_RELEASE_FLOW) to provide a path to removing the non-namespaced variable.
• (Perhaps) rename the jobs to start release-it:... to guard against name clashes. This might be considered a breaking change if downstream configurations rely on the release it job names.