`OAuth2AuthorizationExchange` is not serializable

[marcusdacoregio]

The OAuth2AuthorizationExchange class should be serializable since it is used as a field of OAuth2AuthorizationCodeAuthenticationToken, which in turn implements Serializable, requiring all its fields to be serializable too.

@Test
void oauth2AuthorizationExchangeShouldBeSerializable() throws IOException {
	OAuth2AuthorizationExchange exchange = TestOAuth2AuthorizationExchanges.success();
	try (ByteArrayOutputStream baos = new ByteArrayOutputStream();
		 ObjectOutputStream objectOutputStream = new ObjectOutputStream(baos)) {
		objectOutputStream.writeObject(exchange);
		objectOutputStream.flush();
	}
}