ADSBEx: Support RapidAPI-based endpoint

[TwinFan]

Current Situation / Problem
ADS-B Exchange is going to offer a subscription-based API based on the RapidAPI service.

Suggested Solution

• Support longer key format, the RapiAPI key is really long...and I don't find documentation if it has a fixed length, so we just remove the length restriction.
• Try to support both endpoints: The old key (fixed length with dashes) goes to the old endpoint, the new key to the new endpoint.
• The new endpoint at RapidAPI requires two HTTP headers to be added: X-RapidAPI-Host and X-RapidAPI-Key
• RapidAPI provides the number of remaining requests in the HTTP header X-RateLimit-Requests-Remaining. Let's try displaying that figure.

Response should be the same, so there should be no differentiation and no change to response handling and decoding.

Benefits
Support for ADSBEx also for non-feeders.

Ultimately, feeders shall migrate to RapidAPI as well. They will be given a free service plan there.