Add Starlark as an engine

[apeabody]

Describe the solution you'd like
With the addition of multi-engine(#2616), it would be wonderful if Gatekeeper can add support for Starlark. Starlark implements a subset of the Python language with familiar and readable syntax, and the interpreter is available as a Go module for parallel execution.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 14 days if no further activity occurs. Thank you for your contributions.

[maxsmythe]

keeping alive b/c I'm curious to see what kind of feedback we get here

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 14 days if no further activity occurs. Thank you for your contributions.