Create a more useful example rego file for ec init policies

[mbestavros]

The existing Rego included when running ec init policies is fairly basic. We should include something with a little more detail - particularly something that verifies attestations.

Some potential places to look for inspiration include:

• input.image
• input.attestations

A/C:

• Running ec init policies generates a Rego file that checks an image's attestations.