Remove &mut self in add_credentials

CHANGELOG.md

@@ -6,6 +6,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ### Added
 ### Changed
+- The `ClientAgentSigner` function `add_credentials` no longer takes `self` as mutable. This wasn't required by the 
+  function implementation.
+
 ### Fixed
 ### Removed
 

src/signing/client_signing.rs

@@ -35,7 +35,7 @@ impl ClientAgentSigner {
         }
     }
 
-    pub fn add_credentials(&mut self, cell_id: CellId, credentials: SigningCredentials) {
+    pub fn add_credentials(&self, cell_id: CellId, credentials: SigningCredentials) {
         self.credentials.write().insert(cell_id, credentials);
     }
 }