release/oci: add Oracle Cloud image builder

Provides an OCI (Oracle Cloud Infrastructure) release target for
Oracle's KVM-based VM implementation. Tested using 13.1-RELEASE,
primarily on Ampere CPU on A1.Flex VM shapes, but also works on
amd64 shapes.

• supports cloud-init and custom scripts
• provides a freebsd@ sudo-enabled user
• root user disabled over ssh & console

Approved by: gjb
Reviewed by: emaste
MFS after: 1 week
Sponsored by: The FreeBSD Foundation
Sponsored by: SkunkWerks, GmbH
Technical assistance from: Oracle
Differential Revision: https://reviews.freebsd.org/D34746