Add ACL JSON list output option

[krockpot]

Add a mechanism for writing the ACL out as a JSON list, so it can be fed into knox access -acl

This request came from an internal team.

Tested with a new build locally:

❯ bazelisk run .../knox_client -- acl -json jkrach:test:demo_key                                                                                                                                                                                                                                                                                        ...
[{"type":"User","id":"jkrach","access":"Admin"},{"type":"UserGroup","id":"security-team","access":"Admin"},...{"type":"Machine","id":"cli-testing","access":"Read"}]