feat(mcm): new method has_pending_mapping

[Ant-hem]

Please add the following method to search_client

function has_pending_mappings(retrieve_mappings: boolean = false, opts: request_options) 
return has_pending_mappings_response

struct has_pending_mappings_response {
	pending: boolean,
	clusters: map<string, string[]>
}

{
  "method": "GET",
  "url": "/1/clusters/mapping/pending?getClusters={retrieve_mappings}",
  "hostType": "read"
}

[aseure]

As discussed with @nunomaduro we won't implement this as part of the v1.