Add HEAD endpoints for assets

**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.