Add new "MapAction" overloads

[halter73]

Fixes #30448.

By adding overloads to existing built-in IEndpointRouteBuilder extension methods, we remove the need for the routing attributes ([HttpGet("/"], [HttpPost(..., etc.) in front of most "MapAction" methods. This will be especially handy when the methods become lambdas in non-trivial cases.

namespace Microsoft.AspNetCore.Builder
{
    public static class EndpointRouteBuilderExtensions
    {
        public static MapActionEndpointConventionBuilder MapGet(this IEndpointRouteBuilder endpoints, string pattern, Delegate action);
        public static MapActionEndpointConventionBuilder MapPost(this IEndpointRouteBuilder endpoints, string pattern, Delegate action);
        public static MapActionEndpointConventionBuilder MapPut(this IEndpointRouteBuilder endpoints, string pattern, Delegate action);
        public static MapActionEndpointConventionBuilder MapDelete(this IEndpointRouteBuilder endpoints, string pattern, Delegate action);
        public static MapActionEndpointConventionBuilder Map(this IEndpointRouteBuilder endpoints, string pattern, Delegate action);
        public static MapActionEndpointConventionBuilder Map(this IEndpointRouteBuilder endpoints, RoutePattern pattern, Delegate action);
}

This leaves MapAction for now. I'll have a PR that removes that and IRoutePatternMetadata soon.

[ghost]

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[JamesNK]

Aren't there already Map* extension methods on route builder? Do these not colide because of Delegate parameter?

[davidfowl]

Nope.

[halter73]

These are overloads, but they should only be used for delegates that aren't RequestDelegates. This was also mentioned in dotnet/csharplang#4451 (comment).

[pranavkm]

Do we need this? As a user, there are alternate ways to re-use the delegate if we need to. I'm also similarly against allowing more than one HTTP attributes per MapAction delegate.

[halter73]

I agree that the flexibility of delegates means that we shouldn't need to make routing metadata overly complicated. That's why I removed IRotePatternMetadata and IRouteOrderMetadata in #30563. I also stopped implementing IHttpMethodMetadata in attributes which caused multiple instances to show up on endpoints that didn't have multiple instances before. #29878 (comment)

All that said, this is an overload that already exists for RequestDelegate and any argument about reusing the Delegate, you could make for reusing the RequestDelegate. I'm just keeping it consistent by adding a Delegate overload for every RequestDelegate overload.