Port CodeLens endpoint to cohosting

[davidwengier]

We need to add support for codelens to cohosting, for VS Code.

• Create a new CohostCodeLensEndpoint in the Microsoft.VisualStudioCode.Extension project, next to the other cohost endpoints in the Endpoints folder
• Create a new IRemoteCodeLensService interface in the Workspaces project
• Create an entry in Services.props for the new service
• Create a new RemoteCodeLensService implementation in the Remote.Razor project
• In the endpoint, use the RemoteServiceInvoker to call into the remote service
• In the remote service, call into Roslyn to get the results, then map them from C# to Razor using the Document Mapping Service
• Write tests in the Microsoft.VisualStudio.LanguageServices.Razor.Test project, copying the form of the existing CohostWrapWithTagEndpointTest.cs tests