Multiple changes building on top of each other -- recommended to review commit-by-commit:

• Switches to HotReloadMSBuildWorkspace introduced by HotReloadMSBuildWorkspace roslyn#81577 -- allows us to remove OOP build host and avoid extra design-time build.
• Implements support for file based programs:
  • Adds --file command line switch
  • Provides custom ProjectInstance factory to ProjectGraph loader, which synthesizes project for .cs file.
• Uses msbuild BuildManager type to parallelize DTB
  • Improves loading of a graph of 108 projects from 202s to 17s on 48 core CPU.
• Allows loading project graphs with multiple roots to support polyglot Aspire, where the app host doesn't have references to resource projects (resource projects are the roots of the graph).
• Improves auto-restart logic to restart projects that do not support Hot Reload.
  • E.g. AOT trimmed projects, F# projects, projects with TFM < 6.0, projects that fail to establish browser refresh connection, etc.)
• Make arguments to log messages strongly typed (MessageDescriptor<TArgs>).
  • To avoid mistakes when passing arguments
  • To allow future polyglot Aspire impl to define ILogger that forwards select status messages to Aspire controller
• Fixes a race condition that occurs when shutdown is requested after the process is launched but before the newly created RunningProject is published.
• Fix race condition that can occur when shutdown is requested while Aspire session is being started.
  • The Aspire service factory may be disposed while a session is being started, before the session object gets registered. The newly created process gets orphaned and doesn't exit before the dotnet watch process terminates. This causes a test that validates termination of all processes to hang.

Fixes #51469