Add top level flink-runtime directory to flink autolabeler config

[kbendick]

This adds the upcoming flink-runtime module, and associated top level directory, to be tagged under flink like the current spark-runtime and associated top level spark directories / different spark modules are all tagged as spark.

The pending PR which adds the flink-runtime module is here: #1423

This closes this issue: #1448

[kbendick]

cc @openinx as you are the one who is adding the flink-runtime module in the PR mentioned, but it's not currently labeled flink.

cc @rdblue (sorry that you probably get CC'd on like 75% of PRs), as you were the one who noticed the original bug on the above mentioned flink-runtime PR as it didn't properly tag the updates to build.gradle with the build label. I figured I'd proactively add flink-runtime to flink while I was making these changes.

[rdblue]

I'm merging this now, since it doesn't depend on #1423. Thanks for fixing this, @kbendick!

[kbendick]

I'm merging this now, since it doesn't depend on #1423. Thanks for fixing this, @kbendick!

I'm not 100% sure if the labels will be added to that PR once another push is made or if the autolabeler only runs on PR opening. Either way, it will add the flink label to anything that goes in the upcoming flink-runtime directory going forward. As always, happy to be able to contribute any way that I can @rdblue.