[LSP] Support LSP services associated with LSP server instances (with lifetimes that match).

[dibarbet]

Resolves #61002

This PR creates the concept of an ILspService and ILspServiceFactory. These can be exported via two different attributes

1. ExportStatelessLspService attribute exports an ILspService that must be stateless. The instance exported is re-used across all LSP server instances throughout the lifetime of the process.
2. ExportLspServiceFactory exports an ILspServiceFactory. The factory instance is shared across all servers, however the ILspService creation method is called for each individual server instance. This allows the service to have an instance per server. The services exported via the factory are also disposed of on server shutdown.

Each of the attributes specifies

1. the concrete type that implements ILspService - used to find the service via GetRequiredService<T>()
2. the MEF contract name - used to ensure we only import services for C# when we're in C#, only Xaml when in Xaml, and only JS when in JS to avoid RPS regressions.
3. An optional server kind parameter - used if an lsp server should only apply to a specific LSP server (for example you want to provide a service only for the C# razor server but not the normal C# server).

The majority of the code changes is just a bunch of boilerplate switching all the request handlers and a few other things to use the lsp service concept.

RPS val - https://dev.azure.com/devdiv/DevDiv/_git/VS/pullrequest/399013

[dibarbet]

@sharwell I wanted to get your MEF expertise on this to see if this was the best way to do this. Basically what I want is to be able to create new instances of the ILspService for each LSP server instance (could be multiple running at one time), dispose of them on shutdown, and get new instances if the server is restarted.

The best way I could come up with is making all exports for ILspService not shared and manually retrieve them on server creation here to force mef to create new ones.

However this will require me to suppress #RS0023 // Parts exported with MEFv2 must be marked with 'SharedAttribute' as these cannot be shared instances. I think I can write a diagnostic suppressor for that that only applies to this type perhaps? But wanted to check with you first if you had a better approach.

I also had to import the VS mef ExportProvider then send it down to the LSP layer with AsExportProvider (see LspServiceProvider)

[sharwell]

Rather than export ILspService, you can export ILspServiceFactory. This allows the factory to be static, and also allows the factory to cache the service instance if it wants to. See IWorkspaceServiceFactory for another example.

[dibarbet]

basically did this - there is some extra boiler plate with this approach (e.g. https://github.com/dotnet/roslyn/pull/61266/files#diff-e49357a9ef274059c786a87abe0c688392ac8101a18dc41327d1aaa347c17243R20 ) but the majority are stateless and so don't need to actually use a factory.

To indicate which is which I have two attributes - one for exporting stateless LSP services, and one for exporting lsp service factories

[dibarbet]

running RPS before requesting further reviews - https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=6148840&view=results

[dibarbet]

moved into its own file, unchanged.

[dibarbet]

all this was moved unchanged into this file from a previous partial

[dibarbet]

all this was moved unchanged into its own file

[dibarbet]

now this can be initialized in the ctor instead of on first request

[dibarbet]

new interesting code

[ryanbrandenburg]

Some small questions and comments but it looks good. Probably you'll want a review from someone with more Roslyn background though.

[dibarbet]

merging before there are merge conflicts since this touches a lot of files

if there is more feedback, will take do in a followup