Add commands for managing object storage user policies

[kangasta]

We should add support for managing object storage policies to allow users to configure access to an object-storage instance without need for external tools. See also #570.

We could first add attach (API docs), detach (API docs) and list commands to support using system defined policies. Later also create and delete commands could be useful to enable managing user defined policies as well. See upctl object-storage network implementation for example on how similar commands have been implemented.

[mgajda]

Needs also policy attachment and modification. This can be currently made manually:

curl -X POST "https://api.upcloud.com/1.3/object-storage-2/<service-uuid>/users/${prefix}user/policies" \
  -H "Authorization: Bearer ${UPCLOUD_TOKEN}" \
  -H "Content-Type: application/json" \
  -d '{"name": "ECSS3FullAccess"}'