Support subdirectories in bucket object name

[vilozio]

It would be nice to somehow indicate in which subdirectory the source zip file should be placed.

For example, I have a single bucket for all cloud functions sources, and I want to keep a file layout like this:

my-bucket/
    function1/hash-build.zip
    function2/hash-build.zip

My suggestion is to add an optional variable for object directory path.

[morgante]

This seems like a reasonable suggestion, happy to review a PR adding it.