As noted in #94, release-it was not yet present in the common pipeline
which was a blocker of the vision of "OpenAPI client generation releases
are zero-configuration". #94 also noted that not much had to be done to
add release-it to the common pipeline.

Gate the release-it jobs behind the presence of a `.release-it.json`
file in the root of the repository. This should have zero effect on
current users since that file needs to be present in order to use
release-it. Guard against inadvertent triggering by allowing jobs to be
explicitly disabled via a `RELEASE_IT_DISABLED` variable.

To guard against future namespace clashes, namespace all the release-it
jobs. In #94, it was originally suggested that the
`USE_MERGE_REQUEST_RELEASE_FLOW` variable could be namespaces and given
the backup value of `USE_MERGE_REQUEST_RELEASE_FLOW` to avoid breaking
clients but, upon reflection, having *two* variables controlling this
behaviour adds more confusion than it prevents. As such, leave the
variable un-namespaced as an interesting historical remnant.