remote: Add "ostree remote summary" command

Downloads and prints a remote summary file and any signatures in an
easy-to-read format, or alternatively with the --raw option, prints
the summary GVariant data directly.

https://bugzilla.gnome.org/show_bug.cgi?id=759250

tests/test-pull-summary-sigs.sh

@@ -103,3 +103,14 @@ cd ${test_tmpdir}
 repo_reinit
 ${OSTREE} --repo=repo pull origin main
 echo "ok pull delta with signed summary"
+
+# Verify 'ostree remote summary' output.
+${OSTREE} --repo=repo remote summary origin > summary.txt
+assert_file_has_content summary.txt "* main"
+assert_file_has_content summary.txt "* other"
+assert_file_has_content summary.txt "* yet-another"
+assert_file_has_content summary.txt "found 1 signature"
+assert_file_has_content summary.txt "Good signature from \"Ostree Tester <test@test.com>\""
+grep static-deltas summary.txt > static-deltas.txt
+assert_file_has_content static-deltas.txt \
+  $(${OSTREE} --repo=repo rev-parse origin:main)