[FLINK-18069] [scala, docs] Scaladocs 2.12 fails to recognize inner interfaces

[tzulitai]

This is a similar issue as reported here: scala/bug#10509.

This seems to be a problem with Scaladocs 2.12.x. The only workaround, it seems from the issue reports, is to redundantly add the full-length qualifiers for such interfaces.

Verifying this change

Verified this by doing the following steps

• Build the project: mvn clean install -DskipTests
• Build the Javadocs: mvn javadoc:aggregate -Paggregate-scaladoc
• Build the Scaladocs: cd flink-scala/ && mvn scala:doc

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[rmetzger]

+1 to merge

[tzulitai]

Thanks, merging ...