Remove channel prompt from `aspire new`, `aspire add`, and `aspire update`.

[mitchdenny]

We want to streamline the experience around the aspire add, aspire update and aspire new experiences and remove the confusion that people have around the default feed, the stable feed and other feeds.

Instead all of these tools will by default select the default channel unless any of the following is true:

1. There is a --channel switch that specifies a specific channel.
2. There is a channel configuration value in the app settings.

In the case of aspire new and aspire update ... if there are any hives on the developer machine this means that this is probably a dev team machine and in that particular case it should present the channel selection options (note the goal here is not to remove the channel selection from the code, its just to skip over it if we can).