servlet: historical flight API

[evacchi]

Queries flight APIs for info about past flights,
using the following services:

• https://opensky-network.org
• https://adsbdb.com
• https://airport-data.com

Note:

In OpenSky, Flights are updated by a batch process at night,
i.e., only flights from the previous day or earlier are available using this endpoint.

OpenSky requires a username, password pair, you can easily get
with a free account.

Learn more in the README.

---

Combined with the time tool:

[evacchi]

uhm it looks like the change in schema now requires at least one properties (I was using oneOf, no properties element). I am moving one element to properties so that the schema validates, but that is not strictly necessary