Add `rego_version` attribute to bundle manifest (#6579)

Adding a global `rego_version` attribute to bundle manifest, to inform OPA runtime about what rego-version (v0/v1) to use to parse/compile contained Rego files.
The rego-version of individual Rego files can be overridden through the `file_rego_versions` manifest attribute.

Implements: #6578

Signed-off-by: Johan Fylling <johan.dev@fylling.se>