Add machineTag capturing the extensions used in the dataset

There isn't any easy way to find datasets or occurrences making use of a specific extension currently. It would be a good start to tag datasets with the extension they use. That could also be a starting point for finding the occurrences.

something along ? :
```
namespace: gbif
key: extension
value: factsAndMeassurements

namespace: gbif
key: extension
value: identifiers
```