feat: channel add command

[baszalmstra]

Adds the pixi channel add command to add channels to a project and regenerate the lock-file in one go.

[wolfv]

We could also consider a "project add channel xxx" and "project add platform xxx" subcommand

[baszalmstra]

I had a discussion about this @ruben-arts as well. I see pixi channel add as an analog to git remote add. Why do you prefer to add an additional subcommand (e.g. project)?

[wolfv]

I was mainly thinking about not cluttering the global subcommands too much. Also it would match well with some other project settings like subdirs/platforms.
But I also get your argument

[ruben-arts]

I have to say I cant remember the last conversation we had about this but what if we do the pixi project channel add xxx and then open a "good first issue" for contributors to extend the functionality with the rest of the project info?

Because I think this feature is also nice for the platforms and the version for instance. But adding a pixi platform add and a pixi version add seems to clutter the main cli to me.

[baszalmstra]

Yes correct! We decided to put them under the project subcommand.

[ruben-arts]

Can be closed as it was moved to a project subcommand in #347 .