code: @ConditionalOnSingleCandidate(CallCredentials.class)
      @Bean
      StubTransformer stubCallCredentialsTransformer(final CallCredentials credentials) {
          return CallCredentialsHelper.fixedCredentialsStubTransformer(credentials);
      }
docs: Creates a {@link StubTransformer} bean that will add the call credentials to the created stubs.
  
  <p>
  <b>Note:</b> This method will only be applied if exactly one {@link CallCredentials} is in the application
  context.
  </p>
  
  @param credentials The call credentials to configure in the stubs.
  @return The StubTransformer bean that will add the given credentials.
  @see AbstractStub#withCallCredentials(CallCredentials)
  @sse {@link CallCredentialsHelper#fixedCredentialsStubTransformer(CallCredentials)}