[magics] Allow `%ai list` to take a provider argument

[dlqqq]

Problem

In #1437, %ai list currently just returns every single model. Last time I checked, this is around 1000 models.

Proposed Solution

1. %ai list should list all providers by default, and ask the user to run %ai list <provider-name>.
2. %ai list <provider-name> should list all models available from one provider. It should also note that the list is not comprehensive, and include a reference to the upstream LiteLLM docs.
3. %ai list all should list all ~1000 models as done previously.

Additional context

[srdas]

Closed via PR #1458