Java typed-decoders features

[S11001001]

There is now a collection of associated ledger client facade issues, all somewhat loosely dependent on #14313 (though truly only ~interface FromValue~ interface ValueDecoder is required for all of them).

• [x] on the querying side, ACS and transaction streams: #14969 needs very little of decoders for templates, since TemplateCompanion already carries decoders. It's a little trickier, but interface views can also be incorporated with #15125
• [x] on the choice metaprogramming side, there is #14329. The basics can be done without decoders, but full support for decoding choice arguments and return types needs them
• [x] ChoiceMetadata may also be a good basis for #14312. However, we have to seriously discuss what we want to break with that change
• [x] #13768 does not need anything from value decoders.
  • [ ] However, we can use this for the very similar #12897. We don't need to generate any code for #12897, since it can be derived from the values introduced in #13768 if we also have #14329
    • [ ] this would also close #14421
• [x] #15172

[S11001001]

While there is ever-increasing overlap between the untyped-value classes like CreatedEvent and their typed-value counterparts like Created and Contract<...>, we won't unify them as part of this metaissue.

Additionally, as of this writing our only plans regarding this metaissue are to finish the exercise event ideas included above, and beyond that allow changes to quickstart-java and downstream help to discover new features, at least until we have another idea.