add namespace autocompletion

[particledecay]

What? (description)

Add autocompletion capabilities to the namespace command.

Why? (reasoning)

Right now you can autocomplete available contexts. Namespaces would be the next logical step, though that involves the additional requirement of authenticating to the Kubernetes apiserver. Luckily, in order to switch namespaces you must already have a context enabled, so we should be able to use that context to communicate with the apiserver to retrieve namespaces.

Acceptance

Check your PR for the following:

• you included tests
• you linted your code
• your PR has only one commit (interactive rebase!)
• your commit message follows Conventional Commit format
• you are not reducing the total test coverage