Add ILspSolutionProvider in lsp

[dibarbet]

Depends on #44775

Currently the LSP server uses the VisualStudioWorkspace current solution to retrieve documents. This generally works in solution open cases. However, when only a folder is opened, the LSP server is not able to respond as it can't find documents that are part of another workspace's solution (e.g. misc files). This is something we noticed recently when the solution open experience was not working.

The goal of this change is to allow LSP requests to be served for open folder in cloud envs (currently opened .cs files have no completion, no navbar, no goto def, etc). This change makes it so LSP requests can be served with an LSP misc files experience, like having references to mscorlib in completion.

The change being made here is to create a solution provider that retrieves the correct solution.
Long term we would like the request to come in with a solution snapshot which can be used to find the actual solution. Since we're not there yet, we just use the VS workspace and misc files workspace current solution so that we have at least some support for the open folder scenario in cloud env. Note that I have not added metadataassourceworkspace support as there are issues outside of roslyn (our LSP server isn't requested for those files at all).

[dibarbet]

just moved from the old base class

[CyrusNajmabadi]

This PR is quite large. WHen you make it non-draft, can you explain what's going on, and possibly break into smaller pieces?

[dibarbet]

This PR is quite large. WHen you make it non-draft, can you explain what's going on, and possibly break into smaller pieces?

@CyrusNajmabadi I've updated the description, hopefully that explains it a bit better.
I don't think it will be easy to split into smaller pieces - this is basically adding the ILspSolutionProvider that I need to thread through all the LSP handlers. The majority of the changes are just add/removing parameters related to it and fixing up retrieval of the solutions.
I also did some deletion of a couple of unnecessary pieces that I would have otherwise had to thread through also.

[dibarbet]

@CyrusNajmabadi slightly smaller now! Ready

[jasonmalinowski]

Only real request here is to switch around the arguments for the extension methods: it makes sense to me the 'this' arugment is ILspSolutionProivder and not something like TextDocumentIdentifier. If that method wasn't an extension method we'd probably put it on ILspSolutionProvider directly.

The question about whether we were better off keeping things decoupled and passing in an ILspSolutionProvider to the request handlers rather than MEF I'm happy to defer in a later PR if we want to get this in sooner.