Add new "MapAction" overloads

[halter73]

1. Don't call the overloads "MapAction". Add overloads to the existing MapGet/MapPost/MapDelete/Map extension methods. We should remove the existing MapAction method.

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

2. Create overloads without the string pattern parameter for people you still want to use RouteAttribute. This should throw an InvalidOperationException if there's no route attribute.

namespace Microsoft.AspNetCore.Builder
{
    public static class EndpointRouteBuilderExtensions
    {
        public static IEndpointConventionBuilder Map(this IEndpointRouteBuilder endpoints, Delegate action);
    }
}

[halter73]

Sadly, this will probably cause some code that works today to break at the source level. The compiler would be unable to infer the parameter type is HttpContext in the following:

app.MapGet("/", ctx => ctx.Respsone.WriteAsync("Hello, World!"));