[darc/maestro++] Default channel update

[mmitche]

Add an API which allows for a default channel to be updated, rather than having to delete and recreate the default channel.

The basic rule is that a channel may only be produced from one branch, so updating or adding a default channel association where the channel was previously produced from another branch should update the existing entry.

[riarenas]

Moving to the improvements epic. This looks cheap to implement and we could get to it in the .NET 5 timeframe, but doesn't seems like a priority.