TagTranslator misses functionality to resolve OSHDBTagKey

### Problem Description

The TagTranslator provides a function to get from `OSMTagKey` or `String` to `OSHDBTagKey` but not the other way around

### Expected Solution

Add `lookupTagKey`

### Alternative Solutions

Make `lookupTag` return "all it knows" i.e. an `OSMTag` with a key but Null value. This would allow this function to serve the above information.