Add selectors to 'list' and 'delete' subcommands

Current situation

The public endpoints often support selectors. This should be exposed by opentf-ctl related sub-commands.

Desired outcome

The relevant sub-commands should allow for --selector and --field-selector options.

The following sub-commands are candidates for those enhancements:

• get subscriptions
• get channels
• get agents
• get workflows
• get workflow (already implemented and documented)
• run workflow (already implemented and documented)
• get namespaces (already implemented and documented, but not for field selector, as there is nothing to filter on yet)
• kill workflow
• delete subscription
• delete agent