Support listing operations grouped by tags

[tlmii]

The OpenAPI spec allows for tags on operations, intending them to be used for grouping by tools like ours:

Spec:

A list of tags for API documentation control. Tags can be used for logical grouping of operations by resources or any other qualifier.

This shows up in other tools (see https://github.com/dotnet/HttpRepl/issues/453#issuecomment-738954753) but not in HttpRepl.

At first glance, I'm not entirely sure whether this should be a default (if present) or an option on ls or what. But it seems like something we should support, especially for parity with the ui tools.