Use GetClientAssertionAsync in PAR

Author: josephdecock

src/OidcClient/AuthorizeClient.cs

@@ -139,7 +139,7 @@ private async Task<PushedAuthorizationResponse> PushAuthorizationRequestAsync(st
                 ClientId = _options.ClientId,
 
                 ClientSecret = _options.ClientSecret,
-                ClientAssertion = _options.ClientAssertion,
+                ClientAssertion = await _options.GetClientAssertionAsync(),
 
                 Parameters = CreateAuthorizeParameters(state, codeChallenge, frontChannelParameters),
             };