Define v2 endpoints_client as libarry

Image v2 API's endpoints_client has good interfaces now,
then this patch makes the client as a library.

Partially implements blueprint consistent-service-method-names

Change-Id: Ia1cbb7f9bdd37384686e40c3076b44a9db488364
diff --git a/releasenotes/notes/identity-clients-as-library-e663c6132fcac6c2.yaml b/releasenotes/notes/identity-clients-as-library-e663c6132fcac6c2.yaml
new file mode 100644
index 0000000..b7850d0
--- /dev/null
+++ b/releasenotes/notes/identity-clients-as-library-e663c6132fcac6c2.yaml
@@ -0,0 +1,9 @@
+---
+features:
+  - |
+    Define identity service clients as libraries
+    The following identity service clients are defined as library interface,
+    so the other projects can use these modules as stable libraries without
+    any maintenance changes.
+
+      * endpoints_client(v2)