Feature/convop ngsi9 post context entity types

### Description
NGSI9 Convenience Operation "Post /ngsi9/contextEntityTypes/{typeName}"

Complete with unit tests and a simple harness test

Note that no convenienceMap function is added.
Instead, the RegisterProviderRequest is transformed into a RegisterContextRequest and the service routine postRegisterContext is called (instead of calling a convenienceMap function).

This was "necessary" as registrations are complex and lots of duplicated code is avoided doing this.
Perhaps all convenience operations should do the same ...