Add endpoint for resources to check if they exist

Add a GET endpoint under e.g., `/datasets/exists/v1/{identifier}` which only returns whether or not the asset with that identifier exists (the asset should be public or the user should have read permissions). As it should be available for all resources, it makes sense to add this to the resource router.

Suggestions is based on [#409](https://github.com/aiondemand/AIOD-rest-api/issues/409#issuecomment-2592007885).