Add possibility to pass arguments in {id} parameter in REST api

Hi!
Thanks for the patch. For this feature, I had in mind to use features of the token module. But when I implemented that part, the token service was not part of the storage client services and it has recently been added.

Wouldn't it be more powerful to just support token replacement as it would go beyond static values provided by the query as it could include dynamic values (such as current used login for instance, or a user token, etc.)?

So I would prefer to keep using $this->entityTypeManager->getStorage('my_external_entity')->load("1234"); (to only keep a real id as parameter and keep the existing logic) and provide "color=blue" using another more generic mechanism in the single query parameters part (ie. ::getParametersFormDefaultValue('single')). I would see something either using tokens (which could return nothing if not available for instance) or using an event.

After rethinking of your use case and looking at the code, I don't believe you're trying to achieve things the right way.
I think the current REST client implementation already provides what is need to do what you want, and the xntt_views plugin could also be an alternative for dynamic filtering (while it still has issues to solve).

I will close this issue as "won't fix" but you may reopen it if you are not satisfied. You may also consider chatting with me on the #external-entities channel on Drupal slack!.