feat: Add accessor for CredentialProvider::auth_token (#523)

* Simplifies CLI's invoke-function method
* Matches other SDKs like [Go](https://github.com/momentohq/client-sdk-go/blob/fcd4e35d6be0bce667e3e9b6001d58fad3e62059/auth/credential_provider.go#L46) and [Python](https://github.com/momentohq/client-sdk-python/blob/26b4bd6081dec0d963471f392ddfdc3c26b2238f/src/momento/auth/credential_provider.py#L121)

Author: kirmar

src/credential_provider.rs

@@ -107,6 +107,11 @@ impl CredentialProvider {
         &self.cache_http_endpoint
     }
 
+    /// Returns the user's auth token for momento HTTP apis
+    pub fn auth_token(&self) -> &str {
+        &self.auth_token
+    }
+
     /// Returns a Credential Provider from the provided API key
     ///
     /// # Arguments