[Unprivileged] Split the macOS vault out into a reusable component for macOS only running as root

[blakerouse]

Describe the enhancement:

The Elastic Agent on macOS needs to work without being able to use the system level key store for the vault. There is a filesystem vault but is currently only used on Linux and Windows. When running the Elastic Agent on macOS in unprivileged mode it needs to use the filesystem fault.

Describe a specific use case for the enhancement or feature:

This will allow the Elastic Agent to work properly on macOS installed with --unprivileged.

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)