Add HEAD endpoints for assets

[andrejridzik]

Describe the Feature
Adding HEAD endpoints for assets will be beneficial when only checking the existence of a specific asset without the need of retrieval of the entire asset JSON data.

An Example
The usage would be the same a the current GET asset endpoints, e.g. GET /datasets/v1/{identifier}

HEAD /datasets/v1/1234
HEAD /ml_models/v1/1234
...

Considered Alternatives
Alternative solution relies on using the GET endpoints and handling the 404 Not Found error.

Additional context
This operation is needed since the assets can be deleted in time and users/services need to be able to verify if a specific asset is still existent.