Dotnetup: Add a way to show available channels

We show some of the available channels in the init walkthrough, and probably a few as examples in the help, but otherwise there's not a good way to find out what channels are available. We should add a way to do so.

I'm not sure what the command to do so should be. `dotnetup list` does something else (lists installed SDKs/runtimes). Maybe `dotnetup sdk show-channels` and `dotnetup runtime show-channels`?

Start by tracing the dotnetup init walkthrough, help output, and existing dotnetup list command to understand how SDK and runtime channels are represented. Decide on a discoverable command that does not conflict with installed SDK/runtime listing, then verify that it reports the available channels for both SDKs and runtimes and add or update the relevant command tests.

Written by the indexing model from the issue text.