Add pkgbase signing keys for FreeBSD 15

These keys were generated by cperciva@ using the AWS Key Management
Service. They will not be used for signing anything other than
FreeBSD 15.x pkgbase repositories.

Keys will be generated for FreeBSD 16 at a later date, and likely in
a different way.

With hat: re
MFC after: 8 hours (needed in 15.0-RC1)
Differential Revision: https://reviews.freebsd.org/D53768

(cherry picked from commit b05f38262b58c1e82ad5f076f2b0b9fe08ddc98b)