grpc api-->declarativeconfig types conversions

[grokspawn]

opm typically functions to make FBC catalogs available over GRPC by converting the FBC to declcfg.DeclarativeConfig, storing/validating as model.Model, and expressing via api.Bundle/Package/Channel and #1161 handled that forward-conversion.

However, the library also supports backwards conversions which should be updated to support the modified types.