Allow setting rust-analyzer path via rust-analyzer.json , .vscode/settings.json

[Ngalstyan4]

Feature description

In VSCode you can specify the path to the rust-analyzer by setting the rust-analyzer.server.path key in .vscode/settings.json:

{ "rust-analyzer.server.path": "~/.local/bin/rust-analyzer-linux" }

This is documented in the rust-analyzer manual, and the relevant implementation within the VSCode extension is here

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In rustaceanvim, there is a similar configuration option: vim.g.rustaceanvim.server.cmd(config definition in source).

However, it would be nice if rustaceanvim picked up custom rust-analyzer binary from settings.json as well.
This is particularly useful when different projects require different rust-analyzer binaries.
Currently we need to manually write some logic to configure per project rust-analyzer via custom $PATH or per-project vim.g.rustaceanvim.server.cmd