Generate route handler-endpoints at compile time via source generator

Currently, RequestDelegates for associated route handler-based endpoints are generated at runtime using LINQ expressions. This is a process that requires a fair amount of reflection. We can leverage source generation to generate these endpoints via static analysis.

Some of this work has been prototyped in uController and will need to be formalized as part of the official project.

Generator Basics

• Add RequestDelegateGenerator scaffold to aspnetcore repo
• Respect EnableRequestDelegateGenerator flag in Web SDK
• Refactor code to support marking GeneratedRouteBuilderExtensions as file class
• Set up code and tests for emitting diagnostics from the generator
• Support for a RouteEndpointDataSource implementation that supports a func for populating metadata
• Support for an RouteEndpointDataSource implementation that allows customizing what RDF is called to produce filtered delegate calls

Invoking Handlers

• Support generating correct delegate type signature for a given handler
• Support emitting correct generic-EndpointFilterInvocationContext for a given filtered invocation

[DamianEdwards]

That’s an idea we’ve discussed in some other contexts recently but deserves its own issue. We’d likely want it to discover more than just routes created via minimal APIs (e.g. Razor Pages, MVC, etc.).

[Youssef1313]

Fix for https://github.com/dotnet/roslyn-analyzers/issues/6229 is merged now. You can still include an explicit reference if you want to use latest previews, but otherwise, it should be good to have the transitive dependency.