Ensure "processing model" is included as a registry entry option

[dlongley]

@wes-smith as you know we'll need to update this with the new table layout (string, url scheme, typed-literal, etc.). But we also need to include that a registry entry indicates a "processing model" and we need to indicate that there is a "default" processing model that every entry uses unless otherwise specified and that refers to the algorithms in this spec (either present or not yet added). The processing model indicates how context-based, auto-generated term IDs are generated, what the core / generic type-encoders are (e.g., for date compression) and so on.

Originally posted by @dlongley in #28 (comment)

[filip26]

+1 to this. The order in which value encoders/decoders are applied is also important. There should be a list of “standard/generic” value encoders/decoders, along with an additional entry in the registry record that allows specifying a list of them; both generic and custom. This also opens the question of how to identify them; perhaps using URIs?

Also, this seems related to #47