Add a migrate/keep-files API

Nobody has claimed this yet.

Right now, users must use script hooks to migrate any state when performing an update. A `keepFiles` or `migration` API could simplify this operation by removing the extra complexity of a script and transforming this into a declarative operation that can be better understood by Trident.

Such API would also assist users in case of a rollback where maybe some migrated files need to be migrated back (e.g. credentials/certificates).

The API could look something like:

```yaml
migration:
# Could be a file or dir
- path: /path/to/important/files
# Optional, assumed to be the same as `path`
migrationPath: /path/to/place/the/files/at
mode: <> # Optional, can be used to override
owner: <> # Optional, can be used to override
group: <> # Optional, can be set to override
mode: <> # how to handle this migration
```

`mode:` would be the more interesting field, some examples of possible values:

- `simple` (default): the file is only copied to the new system
- `bidirectional`: the file will be copied back (and overridden if different) in case of a rollback

No files, tests, or entry points are named. Start by reading the existing update, rollback, and script-hook handling, then compare it with the proposed YAML migration fields; done requires an agreed API and defined copy and rollback behavior.

Written by the indexing model from the issue text.