remote-add: Add --sign-verify=KEYTYPE=[inline|file]:PUBKEYREF

remote-add: Add --sign-verify=KEYTYPE=[inline|file]:PUBKEYREF

Per https://github.com/ostreedev/ostree/issues/2080#issuecomment-623614483

A huge benefit of ed25519 (and ECC in general) is that keys are very
short - short enough that it's completely reasonable to inline
them into a command line argument.

And I think that's a good model; it makes the keys very visible.

For example, someone could easily copy-paste a commandline
argument from a webpage (secured via TLS) that says to run
`ostree remote add --sign-verify=ed25519=inline:KEY`.