Add user-provided version string to DAG version detection

### Body

Let's make it possible for DAG authors to tell us a DAG version string.

We can't use it instead of the version from #42333, but we can use it as an input so folks can force a new version even if we aren't able to detect it.

We should also store it separately so that we can display it to the user - it may be meaningful to them, like a git tag or commit.

### Committer

- [x] I acknowledge that I am a maintainer/committer of the Apache Airflow project.