Add endpoint for resources to check if they exist

[PGijsbers]

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.

[PGijsbers]

This is labelled "good first issue" because the implementation can largely be based on the endpoint which retrieves the asset.