Implement C-SPC preview in lsp-rust-analyzer--select-related-test

[petr-tik]

Use the power of ivy preview (as i got used to it in doom emacs) to overload an interactive runner function with preview functionality.

This PR adds an lsp-rust-analyzer extension to find related tests, which offloads the heavy lifting and user interaction to the helper method mentioned in the issue heaing.

https://github.com/emacs-lsp/lsp-mode/pull/2776/files#diff-c8c9a652e719bd23b565da5b6f417aa8c0d55a256e176e0772bfe850a680c37eR975-R982

This is the lambda that extracts the label from each runnable and discards everything else, including file and location
https://github.com/emacs-lsp/lsp-mode/pull/2776/files#diff-c8c9a652e719bd23b565da5b6f417aa8c0d55a256e176e0772bfe850a680c37eR982

Instead of discarding it, we could use the location to jump to the start of the related test function definition before running it as a preview. I believe the C-SPC is bound to ivy-call-and-recenter in doom-emacs.

Implementation

I am hand wavy about the implementation, but I imagine one could implement some sort of advice to override lsp-rust-analyzer--select-related-test with an ivy-specific extension that binds C-SPC (or a non-doom binding) to a preview function.