Feature request: ability to add custom `InstalledChannels` without editing the `~/.julia/juliaup/juliaup.json` file

[DilumAluthge]

I'd like to be able to add custom InstalledChannels. However, I don't want to edit the ~/.julia/juliaup/juliaup.json file. Instead, my preferred interface would be to create a new file called ~/.julia/juliaup/channels.d/dilum_custom_channels.json. And then juliaup will automatically detect any *.json files in the ~/.julia/juliaup/channels.d/ directory and add those channels.

If multiple channels are detected with the same name (e.g. ~/.julia/juliaup/juliaup.json and ~/.julia/juliaup/channels.d/dilum_custom_channels.json both have a channel named release), juliaup should throw an error.

[davidanthoff]

Can you describe a bit more what the purpose of that would be? What is the scenario this would enable?

Also, what would these channels point to? Isn't the existing link command good enough?