msggen now produces F# request/response from schema.json
files in a similar way to rust.

`importlib.metadata` is only usable in python 3.8+
And using backport described in python-poetry/poetry#273 (comment)
will cause following error in CI.
> "importlib_metadata.PackageNotFoundError: No package metadata was found for msggen"

So lets just keep it simple by not having version information in genrated code.

According to the conversation in ElementsProject#5201,
generated code for 3rd-party generater should not reside in this repository.
Instead, only generators under `msggen` should be included.

This commit adjusts F# generator to follow the following rule of thumb.

1. no native F# code, only generators, so every primitives are now reside in `msggen/fsharp.py` with plain text.
2. no object equivalent to `RPCClient`, only DTOs must be generated. Implementing client is now the consumers duty.