Support 3.9. Socket/Plug

[christian-bromann]

See https://datatracker.ietf.org/doc/rfc8610/

3.9. Socket/Plug

For both type choices and group choices, a mechanism is defined that
facilitates starting out with empty choices and assembling them
later, potentially in separate files that are concatenated to build
the full specification.

Per convention, CDDL extension points are marked with a leading
dollar sign (types) or two leading dollar signs (groups). Tools
honor that convention by not raising an error if such a type or group
is not defined at all; the symbol is then taken to be an empty type
choice (group choice), i.e., no choice is available.

        tcp-header = {seq: uint, ack: uint, * $$tcp-option}

        ; later, in a different file

        $$tcp-option //= (
        sack: [+(left: uint, right: uint)]
        )

        ; and, maybe in another file

        $$tcp-option //= (
        sack-permitted: true
        )