MapHost endpoint routing

### Describe the bug
In endpoint routing today routes are defined around paths. There's an additional constraint you can add for a host, but only after already defining a path. Instead what I want to do is map all requests to a specific host as the primary pivot. This can be done today but it is inelegant (I have to define a catch-all route), and in-efficient (route values would still be extracted).

### To Reproduce
Here's what I have to write today:
``` C#
endpoints.Map("/{**path}", handler).RequireHost(host);
```
Proposal:
``` C#
endpoints.MapHost(host, handler);
```