Add MapGroup

[halter73]

This PR add a new MapGroup() extension method to IEndpointRouteBuilder. This returns a new builder for defining groups of endpoints with a common prefix that implements both the IEndpointRouteBuilder and IEndpointConventionBuilder interfaces. This can be used to add endpoints with the given group prefix, and to customize those endpoints using conventions.

Sample Usage

var app = WebApplication.Create(args);

var group = app.MapGroup("/todos");
group.MapGet("/", (int id, TodoDb db) => db.ToListAsync());
group.MapGet("/{id}", (int id, TodoDb db) => db.GetAsync(id));
group.MapPost("/", (Todo todo, TodoDb db) => db.AddAsync(todo));

var nestedGroup = group.MapGroup("/{org}");
nestedGroup.MapGet("/", (string org, TodoDb db) => db.Filter(todo => todo.Org == org).ToListAsync());
nestedGroup.RequireCors("AllowAll");

// This applies to all endpoints defined on group and nestedGroup
group.RequireAuthorization();

// ...

Fixes #36007

[davidfowl]

I'd like to see one test with MVC/SignalR/Health checks, something that's not the ModelDataSource.

Oh ALSO, I'd like a test that fires the change token for a data source that fires change tokens and it working.

[BrennanConroy]

Just some comment cleanup nits

[davidfowl]

Do you have a test where we call MapGroup multiple times with the same prefix? I know we're not keeping track of the prefix but it shouldn't matter unless you have duplicate routes right?

[halter73]

/backport to release/7.0-preview4

[github-actions]

Started backporting to release/7.0-preview4: https://github.com/dotnet/aspnetcore/actions/runs/2222446298