Move neovim extension MsgPack types support to core-rpc

Currently, `neovim-api` module introduces support for Neovim extension types, meaning to use them in `core-rpc` or other modules which don't depend on `neovim-api`, custom implementation is needed. Besides this, this makes usage of library in some cases needlessly complicated (`ReactiveRpcClient` and `RpcClient` don't support these types by default, meaning custom instances need to be created to be used in `neovim-api` module). `NeovimApis` makes it easier for some cases, but ideally it should not be possible to build these clients without support for neovim types (or should not be the default behavior at least).