(Discussion) Kinesis batch window (MaximumBatchingWindowInSeconds)

[spascou]

I wanted to use batch window option for consuming Kinesis events, but did not find the corresponding setting in Zappa. Decided to implement it.

As the Kinesis Event Source currently used is the one provided by Kappa, I have opened a PR there as well (garnaat/kappa#139).
However, considering the activity level on the kappa repository, I'm thinking of adding it here on a short-to-mid-term basis (see https://github.com/spascou/Zappa/commit/62ca54ab3720eaaf3d27da635b7306f0b46f7c70).

To that end, I created an ExtendedKinesisEventSource in a similar fashion that has been done for SNS.

Are there any comments / suggestions on this? I'll open a PR here if it seems like a good idea.