release/oci: add Oracle Cloud image builder

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