Allow `file.recurse` to merge all existing `source` dirs

[lkubb]

What does this PR do?

Adds a merge parameter to file.recurse. When enabled, instead of selecting the first existing source, merges source directories recursively.

What issues does this PR fix or reference?

Fixes: https://github.com/saltstack/salt/issues/67072 Fixes: https://github.com/saltstack/salt/issues/67766

Previous Behavior

Selects the first source only.

New Behavior

Can merge multiple sources.

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• [x] Docs
• [x] Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• [x] Tests written/updated

Commits signed with GPG?

Yes