VEH-43: Support for Gzipped GPXIngest JSON Strings

As the API can directly ingest a JSON encapsulated GPXIngest object, it may be helpful to add support for accepting this as a GZipped string (to reduce bandwidth utilisation, especially on the weak connections VehManPi units are likely to encounter).

Would either need to auto-detect that the string was gzipped, or prescribe a request variable which must be set to denote that compression is in use.