schema: implement oneOf schema support

schema: implement oneOf schema support

A 'oneOf' schema is basically exactly what a rust `enum` is.
Exactly one of the possible values must match the data.

This should ultimately be the base to allow using the
`#[api]` macro on a newtype style enum as well as using this
schema as a configuration for our section config parser.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Reviewed-by: Lukas Wagner <l.wagner@proxmox.com>