Implement `server` and `default_server`

[wolfv]

Problem description

We have been discussing in https://github.com/prefix-dev/pixi/pull/944 to add a server and default_server option.

These would be used to override the conda.anaconda.org namespace server.

The default_server would be used for pixi init or pixi global install. The server would be stored in the pixi.toml file (to make sure that all users of the pixi.toml have the same configuration and the lockfile resolves to the same URLs).