Add LSP outgoing and incoming calls

[languitar]

Is your feature request related to a problem? Please describe.
I am using telescope as my primary interface for list-like LSP results. One thing that is missing is support for the outgoing_calls and incoming_calls methods provided by the neovim LSP implementation.

Describe the solution you'd like
Having telescope lists similar to what is available for other LSP results would be great.

Describe alternatives you've considered

[kkharji]

Hmmm, I'm having trouble understanding that feature. Could you please provide an example implementation. Thanks

[sudormrfbin]

It shows the external function calls made from inside the current function (outgoing calls) and the places where the current function is being called from (incoming calls, similar to references) - https://stackoverflow.com/a/56010680/7115678

[zeertzjq]

telescope-lsp-handlers.nvim has these two handlers. Maybe this can be a reference.

[Conni2461]

#1484 was merged into the current dev branch

See #1938 for more infos