Add support for RPM customizations

Add support for RPM customizations, which currently allow to import RPM
GPG key from a file installed in the image. This is e.g. done for the
Azure RHUI image type.

Signed-off-by: Tomáš Hozza <thozza@redhat.com>

test/cases/api.sh

@@ -445,6 +445,17 @@ EOF
 )
 export FIREWALL_CUSTOMIZATION_BLOCK
 
+RPM_CUSTOMIZATION_BLOCK=$(cat <<EOF
+,
+    "rpm": {
+      "import_keys": {
+        "files": ["/etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-beta"]
+      }
+    }
+EOF
+)
+export RPM_CUSTOMIZATION_BLOCK
+
 if [ "$TEST_MODULE_HOTFIXES" = "1" ]; then
   if [ "$ARCH" = "x86_64" ]; then
     NGINX_REPO_URL="https://rpmrepo.osbuild.org/v2/mirror/public/el8/el8-x86_64-nginx-20240626"