Add support for global conventions on WebApplication

[captainsafia]

Implements #59755.

This pull request introduces changes to the WebApplication class and its associated components to support global endpoint conventions.

Changes include:

• Replaced the internal _dataSources list with GlobalEndpointRouteBuilder and RouteGroupBuilder to create an implicit global route group for all endpoints (src/DefaultBuilder/src/WebApplication.cs).
• Added a new Conventions property to expose IEndpointConventionBuilder for the application (src/DefaultBuilder/src/WebApplication.cs) that maps to underlying RouteGroupBuilder
• Added test cases in WebApplicationGlobalConventionTests to verify the application of global conventions on endpoints (src/DefaultBuilder/test/Microsoft.AspNetCore.Tests/WebApplicationGlobalConventionTests.cs).

Wait to review + merge until attached API proposal is reviewed.

[captainsafia]

@halter73 I realized while working through test cases for this that route groups are only designed to work with RouteEndpoints due to the requirement to be able to concatenate the route prefix to the route when constructing the endpoint. See the comment you left here below.

aspnetcore/src/Http/Routing/src/EndpointDataSource.cs

Lines 45 to 50 in 31d5a5f

	// Endpoint does not provide a RoutePattern but RouteEndpoint does. So it's impossible to apply a prefix for custom Endpoints.
	// Supporting arbitrary Endpoints just to add group metadata would require changing the Endpoint type breaking any real scenario.
	if (endpoint is not RouteEndpoint routeEndpoint)
	{
	throw new NotSupportedException(Resources.FormatMapGroup_CustomEndpointUnsupported(endpoint.GetType()));
	}

We have two options at the moment:

• Leave things as is. The WebApplication will still work with our primary scenarios (Blazor, MVC, SignalR, etc.) but users using custom endpoints will have to use a branched pipeline to register endpoints.
• Revise the implementation of grouped endpoints to support un-routed endpoints, perhaps by defaulting to an empty prefix + route for all? I haven't evaluated how expensive this would be.

[halter73]

I don't think we can really make WebApplication throw for any non-RouteEndpoints if they worked before, but maybe we could only throw if you've added a global convention.

I don't think it's even feasible to support global conventions for non-RouteEndpoints, because I imagine most non-RouteEndpoint implementations would be some other derived type which we wouldn't be able to instantiate with the additional metadata from the global conventions.

At best, we could create a plain Endpoint with the additional metadata. We could restrict it to support just RouteEndpoint and plain non-derived Endpoint (but only for global conventions and maybe prefix-less route groups), but I'd want to know exactly what the use case is before we expand support.

This also makes me wonder if just adding a parameterless MapGroup overload is sufficient for this scenario. I know it lacks discoverability though.