Define volume extensions_clients as library

Volume extensions_clients have good interfaces now,
then this patch makes them as a library.

Partially implements blueprint consistent-service-method-names

Change-Id: I9d9f1174cd8e13d98ffe1cf22149a60cef5c502a
diff --git a/releasenotes/notes/volume-clients-as-library-9a3444dd63c134b3.yaml b/releasenotes/notes/volume-clients-as-library-9a3444dd63c134b3.yaml
new file mode 100644
index 0000000..101263f
--- /dev/null
+++ b/releasenotes/notes/volume-clients-as-library-9a3444dd63c134b3.yaml
@@ -0,0 +1,10 @@
+---
+features:
+  - |
+    Define volume service clients as libraries
+    The following volume service clients are defined as library interface,
+    so the other projects can use these modules as stable libraries
+    without any maintenance changes.
+
+      * extensions_client(v1)
+      * extensions_client(v2)