Pre-populated packing tables

[miri64]

@chrysn wrote on the CBOR mailing list:

Are the packing tables pre-populated with any values? ("_coap._udp."
and ".com" would come to mind). If so, is the 1 in ;packed=1
intended to serve as an extension point that dispatches the values of
the table?

I really like this idea! However, some details need to be ironed out as this could go several (somewhat disjunct) routes:

1. Are these extension points pre-defined tables?

2. Do we use CRIs/URIs to identify and find tables (see e.g. https://datatracker.ietf.org/doc/draft-amsuess-cbor-packed-by-reference/)?

3. Should the extension point be used in a similar manner as SCHC rule IDs, i.e., identify some pre-shared tables?

4. and 3. don't necessarily contradict each other, as we could reserve number ranges for either pre-defined tables or pre-shared tables? 2. could also be integrated, e.g. with a dedicated number.