Client: add support for policy management

[MonkeyCanCode]

Implementation for policy management via Polaris CLI (#1867).

Here are the subcommands to API mapping:

attach
 - PUT /polaris/v1/{prefix}/namespaces/{namespace}/policies/{policy-name}/mappings
create
 - POST /polaris/v1/{prefix}/namespaces/{namespace}/policies/{policy-name}/mappings
delete
 - DELETE /polaris/v1/{prefix}/namespaces/{namespace}/policies/{policy-name}
detach
 - POST /polaris/v1/{prefix}/namespaces/{namespace}/policies/{policy-name}/mappings
get
 - GET /polaris/v1/{prefix}/namespaces/{namespace}/policies/{policy-name}
list
 - GET /polaris/v1/{prefix}/namespaces/{namespace}/policies 
   - This is default for `list` operation
 - GET /polaris/v1/{prefix}/applicable-policies
   - This is when we have `--applicable` option provided
update
 - PUT /polaris/v1/{prefix}/namespaces/{namespace}/policies/{policy-name}