Provide method to check if OSM tag is used or return list of involved tags

[karussell]

See discussion here:

Alternatively, we could add a new method to the default speed limits library that outputs all tags used by the currently loaded legal speed limits, so that no maintenance of this functionality is needed.

A method might be a bit better suited as one could also test if tag "startsWith" a certain string etc.

[westnordost]

True