HostString does not validate value

### Describe the bug
The `HostString(string value)` constructor does not validate the value. If the value is not a valid `hostname` or `hostname:port`, it would be nice if the `HostString` constructor would throw, instead of just parsing based on the `:` character. This could help prevent unintentional bugs when passing something like a full URL to `HostString`.

### To Reproduce

```C#
var hostString = new HostString("https://example.org");
Console.WriteLine($"hostString.Host: {hostString.Host}");
Console.WriteLine($"hostString.Port: {hostString.Port}");
```

Output:
```
hostString.Host: https
hostString.Port:
```

Expected: Throw an exception.

### Further technical details
- `Microsoft.AspNetCore.Http.Abstractions` version `2.2.0`