composefs-backend: Rename 'composefs-native' to 'composefs-backend'

We were using composefs-native and composefs-backend interchangeably.
Replace all instances of `composefs-native` with `composefs-backend`

Move all composefs-backend options to a single struct so that we can
test for boolean instead of testing for Some/None for composefs-backend
options

Signed-off-by: Pragyan Poudyal <pragyanpoudyal41999@gmail.com>
Signed-off-by: Colin Walters <walters@verbum.org>

tmt/tests/examples/bootc-uki/install-grub.sh

@@ -19,7 +19,7 @@ podman run \
     --security-opt label=type:unconfined_t \
     "${IMAGE}" \
     bootc install to-disk \
-        --composefs-native \
+        --composefs-backend \
         --boot=uki \
         --source-imgref="containers-storage:${IMAGE}" \
         --target-imgref="${IMAGE}" \

tmt/tests/examples/bootc-uki/install-systemd-boot.sh

@@ -24,7 +24,7 @@ podman run \
     --security-opt label=type:unconfined_t \
     "${IMAGE}" \
     bootc install to-disk \
-        --composefs-native \
+        --composefs-backend \
         --boot=uki \
         --source-imgref="containers-storage:${IMAGE}" \
         --target-imgref="${IMAGE}" \