Add license headers and spotless checks for them

It looks as if there are a couple of files that are missing their appropriate license headers, e.g. https://github.com/apache/flink-training/blob/0b1c83b16065484200564402bef2ca10ef19cb30/common/src/main/java/org/apache/flink/training/exercises/common/datatypes/RideAndFare.java

We should fix that by:

1. adding the missing license headers
2. adding spotless checks to ensure this doesn't happen again

Potential downside: if a user doing the training exercises creates files on their own, these would need the license header as well. On the other hand, a simple `./gradlew spotlessApply` can fix that easily