Provide Context about rdm-schema

[peternewman]

Can I start with a meta comment @sammysmallman . From past similar discussions in other repos, I'd say these are numerous separate issues, and so some threading of discussion would be very useful, either with GitHub Discussions (not currently enabled on this repo) and a new top level comment in one thread for each suggestion, or otherwise closing this issue and splitting it into multiple new issues.

I'd suggest the latter, with a meta issue covering the general concept of extensions and probably generally how they'd fit into the schema. Then a separate issue for each extensions, as some like action say may be pretty trivial (the most complex I can think of is the interlocked INTERFACE_APPLY_CONFIGURATION) whereas others like Informs/Informed By could be more involved and hence want more discussion with something like DEFAULT_SLOT_VALUE which is chained of SLOT_INFO and the personality PIDs. The meta issue or yet another issue could also cover something you appear to have at least considered with manufacturer specific extensions to the standard too...