cmd/cue: richer configurability of encodings

[rogpeppe]

The filetypes syntax (e.g. cue+jsonschema:) is nicely concise to use on the command line, but
as encodings become more complex, it is quite limited and does not scale well.
For example, it would be useful to be able to specify URI to package mappings when
importing JSON Schema, but the simple tag=value syntax of filetypes would probably not
be up to that job. We'd end up needing to define quoting syntax etc, but we already have
a config language that is up to the job: CUE itself.

I suspect the right answer is to do something similar to CUE_REGISTRY: support the existing
simple syntax as syntax sugar for a richer format that can be specified in CUE (in CUE_REGISTRY
that's done with file: and we could do something similar).