Automatic restore is slow when there are many un-restored projects

The basic problem is that currently the server shells out to the CLI once per project - https://github.com/dotnet/roslyn/blob/main/src/Features/LanguageServer/Microsoft.CodeAnalysis.LanguageServer/LanguageServer/Handler/Restore/RestoreHandler.cs#L50

This can be quite a lot slower than restoring the entire solution in large solutions (for example Roslyn.sln).

Some potential improvements
1. See if it is possible to restore multiple projects at once via a single dotnet CLI command
2. Detect on the server side if there are many projects needing a restore, and use the solution path instead (if there is one available).