Add version metadata for repo/dvcfiles #4100

It'd be good to add version information about repo to .dvc/config and to the run-cache, so as to in the future, we can provide better error message/validation or even maintain backward-compatibility if we want to.

Also, it's hard to see where dvc.yaml is going, so, maintaining good support without breaking will be tough (if we decide to maintain compatibility). Same with dvc.lock file.

For .dvc files, I think, they are mature enough that it's very unlikely we'll be making any breaking changes to it.