Improve upload UX

[baszalmstra]

Currently it's very vague how to upload a package using pixi upload.

• We should improve the UX on prefix.dev. Provide help text what the proper command should be for different tools.

• Currently you have to use:

  pixi upload https://prefix.dev/api/v1/upload/<channel_name> <filename>
  

  This is user-unfriendly and hard to discover.

• Uploading to any URL that accepts the request is enough for the command to think it succeeded. For instance:

  pixi upload https://prefix.dev/channels/<channel_name> <package> -vvv
  

  Just succeeds but doesn't actually do anything.

[pavelzw]

You could just incorporate the upload CLI from rattler-build and put it into rattler, similar as with auth login.