Add MapGroup (#41265)

src/Http/Http.Abstractions/src/PublicAPI.Unshipped.txt

...tp.Abstractions/src/Routing/EndpointMetadataCollection.cs

src/Http/Routing.Abstractions/src/PublicAPI.Unshipped.txt

...ttp/Routing/src/Builder/EndpointRouteBuilderExtensions.cs

src/Http/Routing/src/GroupRouteBuilder.cs

src/Http/Routing/src/Patterns/RoutePatternFactory.cs

src/Http/Routing/src/PublicAPI.Unshipped.txt

src/Http/Routing/src/Resources.resx

src/Http/Routing/test/UnitTests/Builder/GroupTest.cs

...Builder/RouteHandlerEndpointRouteBuilderExtensionsTest.cs

...Routing/test/UnitTests/CompositeEndpointDataSourceTest.cs

...outing/test/UnitTests/Patterns/RoutePatternFactoryTest.cs

src/Http/samples/MinimalSample/MinimalSample.csproj

src/Http/samples/MinimalSample/Program.cs

src/Mvc/samples/MvcSandbox/Startup.cs

src/Mvc/test/Mvc.FunctionalTests/RoutingGroupsTest.cs

src/Mvc/test/WebSites/RoutingWebSite/StartupForGroups.cs