Add additional roles method to v3 roles client.

Adds the following API call to the roles_client library:
  - list all role inference rules (/v3/role_inferences)

This enables querying these APIs for a list of all the
role assignments or all the role inferences.

Note that listing all role inference rules is distinct
from listing inference rules. This can be confirmed
in the API docs [0] and in the code itself [1]
(see list_role_inference_rules vs list_implied_roles).

[0] https://developer.openstack.org/api-ref/identity/v3/#list-all-role-inference-rules
[1] https://github.com/openstack/keystone/blob/3e5ead0a45f698eed4162787b723090cee4733f8/keystone/assignment/routers.py

Change-Id: I9d6c0dc83a85bbca173b753183d838adfee04008
diff --git a/releasenotes/notes/add-additional-methods-to-roles-client-library-178d4a6000dec72d.yaml b/releasenotes/notes/add-additional-methods-to-roles-client-library-178d4a6000dec72d.yaml
new file mode 100644
index 0000000..01136c6
--- /dev/null
+++ b/releasenotes/notes/add-additional-methods-to-roles-client-library-178d4a6000dec72d.yaml
@@ -0,0 +1,7 @@
+---
+features:
+  - |
+    Add missing API call, list all role inference rules,
+    to the roles_client library. This feature enables the
+    possibility of listing all role inference rules in the
+    system.