Move neovim extension MsgPack types support to core-rpc

[esensar]

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).

[github-actions]

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