Allow configuring clangd path & arguments on a per-project basis

[Kummallinen]

Some toolchains will require the use of different clangds, or specific arguemnts to clangd, tohandle custom language extensions. There should be a way for the clangd & arguments used to be configured on a per-project basis

[ghentschke]

The current (default) implementation uses a single LS for the workspace.
To implement different LS per project the singleton attribute of the org.eclipse.lsp4e.languageServer extension point has to be set to false.
Since the multiple server topic is a special case, the singleton attribute should be part of an interface or class to be implemented by the extension point.

[ghentschke]

e.g. part of the ProcessStreamConnectionProvider

A PR on LSP4E is needed here.

When the singleton attribute is settable via interface/class the toolchain(s) can provide the appropriate LS and settings by implementing the org.eclipse.cdt.lsp.serverProvider extension.

[ruspl-afed]

Project level configuration is done during rework of access to preferences . However, additional effort required to have multiple LS instances per workspace.