Replace shell script with Python

[in0rdr]

The current shell script implementation for the Kubernetes CronJob works for simple scripting purposes, but does not scale well for more complex logic (such as custom retention and maybe other features in the future).

Reasons for Python implementation:

• We don't have to write to a file (we can read the file stream and store the object directly from memory to s3)
• We can get rid of S3_URI input parameter

[in0rdr]

Good first issue if you would like to to do some Python programming. The draft is already testable. Would be good if we keep the VAULT_TOKEN authentication as a fallback (for testing purposes) when no Kubernetes auth credentials are provided as an input.