Implement print built-in function

[anderseknert]

With this now merged into OPA: open-policy-agent/opa#5501

We'll likely need to "rebase" our OPA version to include tests for this (in case there are any in the YAML suite), but once that's done, this should be a fairly trivial addition. Default behavior should be to pipe print calls via the logger, and have printing be configurable separately from the application log events.