Implement StandardMethodCodec for C++ shells

[stuartmorgan-g]

Currently on JSONMethodCodec is supported for the C++ plugin APIs. This requires defining a class structure than can represent the structure on the C++ side, since the standard library collections don't support mixed value types.

[stuartmorgan-g]

The supporting class to create standard-codec-compatible C++ structures is in progress; defining the reader and writer once that's in place should hopefully be straightforward.

[stuartmorgan-g]

Status update: Implementation is complete except for extensibility (which will be a follow-up). I need to do some minor cleanup, and I want to test it out with the FDE plugins just as a sanity check, but I expect PRs to start going out today.

[stuartmorgan-g]

Filed #31174 to track extensibility as a follow-up.