Create `IConfiguration` extension methods to get Aspire convention-based values

It would be really nice if Aspire added some extension methods on `IConfiguration` that allow for convenience-based APIs for getting values.

For example, it's ugly to ask the config for `"services__serviceName__0"`. Something like this might be really useful:

```csharp
public static class ConfigurationExtensions
{
public static Uri GetServiceHttpUri(this IConfiguration config, string name) =>
config.GetServiceUri(name, 0);

public static Uri GetServiceHttpsUri(this IConfiguration config, string name) =>
config.GetServiceUri(name, 1);

private static Uri GetServiceUri(this IConfiguration config, string name, int index)
{
ArgumentException.ThrowIfNull(name);

var url = config[$"services:{name}:{index}"];

return new(url);
}
}
```