Cloud Function for automatic folder inclusion: improvements

[nick4fake]

Follow up of Cloud Function for automatic folder inclusion

Nice to have:

• Add tests
• Failure notifications
• Switch to a native Go library to make terraform a native dependency
• Retries for cloud function
• Add support for subfolders
• Reduce function code zip file
• Possibly simplify whole approach

[morgante]

I've prioritized the items above.

[nick4fake]

@morgante

[morgante]

Moved TODO to #16

[morgante]

@nick4fake I fixed issue 4 via #18, FYI.