Create infrastructure for sample code

[shaunagm]

This is my first stab at creating a new infrastructure for handling sample code. I expect it to need significant edits before merging.

This PR:

• adds a README describing our sample script process, and including a table listing existing scripts
• creates a template_script.py file that outlines how we'd like the scripts to be structure
• edits the existing four scripts to be in line with the template

Some particular feedback I'd like:

• do the label names I've chosen work? once I get the thumbs up, I'll create the labels and update the PR so the label links in the README link to the labels.
• does the structure of the template_script look good?
• does how we're now handling config variables look good? I want to isolate config variables in their own section at the top so they're easy to use. Currently these scripts may overwrite environmental variables, which may not be ideal.
• have I correctly described what each of the four sample scripts does?

Also if anyone can actually test the refactored sample scripts that'd be swell.

[eliotst]

I just had a couple of small comments, and some thoughts for the future as we build the scripts out.

[eliotst]

I think "vendor" to "own" is maybe what the original script was for, but I think the script can be used for transferring data between any two buckets. I think we can generalize this to just "source" bucket to "target" bucket or something.

[eliotst]

If we repurpose as "source" / "destination", we would need to change these environment variable keys.. AWS_SOURCE_SECRET_ACCESS_KEY, AWS_DESTINATION_SECRET_ACCESS_KEY, etc.