Introduce per channel override feeds into configuration.

At the moment the Aspire CLI has a `PackagingService` that returns a set of channels. Each channel has one or more feeds associated with it which point to publicly available feeds. Some enterprises may want to override these feed URLs via configuration.

For example:

```bash
aspire config set -g channels.stable.nugetorg.url https://mycompany.com/feed/aspire/stable
```

We would need to figure out how we define mappings etc as well.