[FEATURE REQUEST]: Support system-installed LSP servers for dotnet

[xgqt]

lsp-mode wants to always install .NET LSP servers like csharp-ls and fsac (fsautocomplete).
It would be nice to detect if required binaries are already in PATH and not install them if they exist.

To workaround this I have to fist install the servers via autoinstall (from M-x lsp) and then link system binaries to ~/.dotnet/tools/TOOL-NAME.

[xgqt]

Looking at #3906 I think the best compromise search chain should be as follows:

• local tool
• "global" tool (actually hosted inside ~/.dotnet)
• system binary

[xgqt]

I installed lsp-mode from HEAD, look like all that has to be done is to set lsp-fsharp-use-dotnet-tool-for-fsac to nil but for csharp-ls lsp-csharp--cls-test-csharp-ls-present still does not allow anything similar.