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

share/keys/Makefile

@@ -1,3 +1,3 @@
-SUBDIR= pkg
+SUBDIR= pkg pkgbase-15
 
 .include <bsd.subdir.mk>

share/keys/pkgbase-15/Makefile

@@ -0,0 +1,3 @@
+SUBDIR=	trusted
+
+.include <bsd.subdir.mk>

share/keys/pkgbase-15/trusted/Makefile

@@ -0,0 +1,8 @@
+PACKAGE=	pkg-bootstrap
+
+FILES=	awskms-15
+
+FILESDIR=	${SHAREDIR}/keys/pkgbase-15/trusted
+FILESMODE=	644
+
+.include <bsd.prog.mk>

share/keys/pkgbase-15/trusted/awskms-15

@@ -0,0 +1,2 @@
+function: "sha256"
+fingerprint: "1d7b45d20fa8d6ed26f9b4a13ac81a6b5df860b9fe644d07b87e92298ba72595"