RFC: Syntax changes to add support of other CTF metadata

The CTF spec defines metadata types enumerations, structures and variants
that are not yet available for use in tracepoints in LTTng-ust. This
proposes the syntax for how those types could be defined and used.

For now, it won't be possible to define them inline in a tracepoint. They
will have to be named and defined outside the TRACEPOINT_EVENT macro and
used in the tracepoint definition. It will make it easier for a first
implementation. Once that is done, it won't be too difficult to add inline
support.

Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>