feature: add option to specify number of layer versions to keep

[yezarela]

Hi, would like to have this feature so users can specify number of layer versions to keep.

This feature is needed to avoid old layer versions to take up much storage which was limited by AWS.
As stated in AWS documentation here https://docs.aws.amazon.com/lambda/latest/dg/gettingstarted-limits.html , there is hard limit of 75GB of uploaded functions + layers storage.

Would like to hear your feedback, Thank you!

Example result:

[agutoli]

Hi @yezarela thank you so much for this PR. It is very welcome. Unfortunately, I'm very busy and rarely have time to work on this project. Thanks again.

So, there's an open issue about this:
#47

It would be awesome if we could change the option's name from keepVersion to retainVersions, which is more similar to the native support.

[yezarela]

@agutoli thanks for responding! sure, i've updated it to use retainVersions.
please kindly check, thanks

[brunocasado]

would be nice have this feature :)

[agutoli]

This feature was already merged.