File version management in profiles: Do we want to do it? How?

This is an issue that came up during the merge of `elastic-package profiles`.

With the addition of user-managed config profiles that aren't directly re-written by `elastic-package`, we have a potential issue where the user can upgrade to a new release, and their existing custom config will no longer work if the release requires upgraded files.

At minimum, we should have some way of telling the user "hey, this file will no longer work."

A few ideas:

- Manually manage individual semver tags for each managed file, then `elastic-package` can warn the user if a file has a newer version.
- Do some git shenanigans during `make build` to mark or tag certain files if the user might need to update them
- Do file comparisons during runtime

These all have their own drawbacks, but they're the first ideas that popped into my head.