Enhance assertions.md with client action diagrams

Added sequence diagrams for client actions in OAuth 2.0 framework.

Author: stianst

seq/assertions.md

@@ -1,4 +1,6 @@
 ## Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants
+
+### Client action on behalf of user
 ```mermaid
 ---
 config:
@@ -16,6 +18,25 @@ sequenceDiagram
     C->>RS: Request with token
 ```
 
+### Client action on behalf of itself
+```mermaid
+---
+config:
+    mirrorActors: false
+---
+sequenceDiagram
+    participant C as Application
+    participant TS as Token Service
+    participant KC as Keycloak
+    participant RS as REST API
+    C->>TS: Request assertion
+    TS->>C: Assertion
+    C->>KC: Client credential request, with client_assertion
+    KC->>C: Token response
+    C->>RS: Request with token
+```
+
+
 ## OAuth Identity and Authorization Chaining Across Domains
 
 ```mermaid