Registering an extension prefix for CoverageJSON

[havardf]

#26 mention extending CoverageJSON through the prefix mechanisme defined in that standard.

We probably should have one prefix registered for all of RODEO / EUMETNET extensions in CoverageJSON.

So, I suggest register the prefix eumetnet using the mechanism described here: https://covjson.org/prefixes/. Which results in extensions in CoverageJSON like this:

"eumetnet:wigosID": "<some wigos id>",

Then, add a rule in the profile document describing how CoverageJSON extension mechanism works, and also listing all the specific extensions covered by the profile.