oauth: Simplify copy of PGoauthBearerRequest

Follow-up to 03366b61d. Since there are no more const members in the
PGoauthBearerRequest struct, the previous memcpy() can be replaced with
simple assignment.

Author: Jacob Champion <jacob.champion@enterprisedb.com>
Discussion: https://postgr.es/m/p4bd7mn6dxr2zdak74abocyltpfdxif4pxqzixqpxpetjwt34h%40qc6jgfmoddvq

src/interfaces/libpq/fe-auth-oauth.c

@@ -781,7 +781,7 @@ setup_token_request(PGconn *conn, fe_oauth_state *state)
 			goto fail;
 		}
 
-		memcpy(request_copy, &request, sizeof(request));
+		*request_copy = request;
 
 		conn->async_auth = run_user_oauth_flow;
 		conn->cleanup_async_auth = cleanup_user_oauth_flow;