Cosmos: Allow to use a custom JSON serializer

[AndriySvyryd]

Allow value converters to output a raw object (e.g. JToken)

Also provide access to unmapped properties (through a JSON shadow property or dynamic property bag).

Make sure that FromSqlRaw parameters are properly serialized.

Test with STJ source generators

Related to #14570, #30677

[ajcvickers]

Note from triage: this is currently blocked on the new Cosmos SDK, tracked by #18753.