clangd: Implement switch source/header extension (#14646)

Release Notes:

- Added switch source/header action for clangd language server (fixes
[#12801](https://github.com/zed-industries/zed/issues/12801)).

Note: I'm new to both rust and this codebase. I started my
implementation by copying how rust analyzer's "expand macro" LSP
extension is implemented. I don't yet understand some of the code I
copied (mostly the way to get the `server_to_query` in `clangd_ext.rs`
and the whole proto implementation).

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Co-authored-by: Kirill Bulatov <kirill@zed.dev>