feat: expose service tokens for Metal connections (#247)

This updates the API response mapper for Metal connections so that
Ansible collection users can access `service_tokens` for a Metal
connection without having to go to the [Equinix Metal
console](https://console.equinix.com).

plugins/module_utils/metal/metal_api.py

@@ -281,6 +281,7 @@ def get_assignment_address(resource: dict):
     'ports': 'ports',
     'requested_by': 'requested_by',
     'status': 'status',
+    'service_tokens': optional('service_tokens')
 }
 
 def private_ipv4_subnet_size(resource: dict):