Fixed RD-15361: Added akka.license-key config that defaults to $AKKA_LICENSE_KEY

[bgaidioz]

With no AKKA_LICENSE_KEY variable set, it goes to dev mode that stops after 15 minutes. (It doesn't pick the empty string as a key that is wrong.)

WARN  akka.actor.ActorSystemImpl - Dev use only. Free keys at https://akka.io/key

With a proper AKKA_LICENSE_KEY value, it works.

INFO  akka.actor.ActorSystemImpl - License check succeeded for Lightbend user (...). License is valid until (...).

With a wrong AKKA_LICENSE_KEY value, the error shows and the DAS immediately stops.

ERROR akka.actor.ActorSystemImpl - Invalid key. Free keys at https://akka.io/key
INFO  akka.actor.CoordinatedShutdown - Running CoordinatedShutdown with reason [ActorSystemTerminateReason]
Exception in thread "main" java.lang.RuntimeException: Invalid key: Error decoding license key: Invalid base62 length