ssh_authorized_key should be able to take a ready-made OpenSSH public key

Use Case

When creating adding an authorized public key with the Ssh_authorized_key-resource, the manifest is supposed to provide all of the pieces of the key.

Describe the Solution You Would Like

It would be very nice, if the resource could simply parse an existing foo.pub in the format used by OpenSSH (maybe, in some other formats as well). The path to the file can be interpreted the same way, source-attribute of the File-resource is or, perhaps even better, the way template() interprets its arguments.

Options specified in the manifest can complement and, in case of conflict, overwrite those found in the file.

By treating the keys as data rather than as code, it would be easier for users to deal with large number of them.

Describe Alternatives You've Considered

N/A

Additional Context

This was originally filed as MODULES-7595.

Start with the Ssh_authorized_key resource and review how its current manifest attributes represent public-key parts. Define how an existing OpenSSH public-key file is supplied, how manifest options complement or override it, and what behavior should result for conflicting values. Done means the resource can consume a ready-made key while retaining explicit option precedence.