Aerodrome refs other than ICAO, IATA, FAA

Given the reasonable comments #1, #2 regarding the IDs of objects in disputed territories, I will add information about the <CountryCode> extension for objects in such territories to the first post.

I will also add an additional clarification that the national official identifier means the identifier that is assigned by the national aviation agency, so that there is no misunderstanding later.

I will also replace ref:<name> with the proposed ref:<agency> so that there is no confusion with the name tag, and I will add information that the agency may also refer to identifiers of official services that are not national aviation agencies, such as TRACES.