pull: Add support for sign-verify=<list>

The goal here is to move the code towards a model
where the *client* can explicitly specify which signature types
are acceptable.

We retain support for `sign-verify=true` for backwards compatibility.
But in that configuration, a missing public key is just "no signatures found".

With `sign-verify=ed25519` and no key configured, we can
explicitly say `No keys found for required signapi type ed25519`
which is much, much clearer.

Implementation side, rather than maintaining `gboolean sign_verify` *and*
`GPtrArray sign_verifiers`, just have the array.  If it's `NULL` that means
not to verify.

Note that currently, an explicit list is an OR of signatures, not AND.
In practice...I think most people are going to be using a single entry
anyways.

tests/test-signed-pull.sh

@@ -23,7 +23,7 @@ set -euo pipefail
 
 . $(dirname $0)/libtest.sh
 
-echo "1..16"
+echo "1..20"
 
 # This is explicitly opt in for testing
 export OSTREE_DUMMY_SIGN_ENABLED=1
@@ -102,6 +102,31 @@ test_signed_pull "dummy" ""
 repo_init --sign-verify=dummy=inline:${DUMMYSIGN}
 test_signed_pull "dummy" "from remote opt"
 
+# And now explicitly limit it to dummy
+repo_init
+${CMD_PREFIX} ostree --repo=repo config set 'remote "origin"'.sign-verify dummy
+${CMD_PREFIX} ostree --repo=repo config set 'remote "origin"'.verification-dummy-key "${DUMMYSIGN}"
+test_signed_pull "dummy" "explicit value"
+
+# dummy, but no key configured
+repo_init
+${CMD_PREFIX} ostree --repo=repo config set 'remote "origin"'.sign-verify dummy
+if ${CMD_PREFIX} ostree --repo=repo pull origin main 2>err.txt; then
+    assert_not_reached "pull with nosuchsystem succeeded"
+fi
+assert_file_has_content err.txt 'No keys found for required signapi type dummy'
+echo "ok explicit dummy but unconfigured"
+
+# Set it to an unknown explicit value
+repo_init
+${CMD_PREFIX} ostree --repo=repo config set 'remote "origin"'.sign-verify nosuchsystem;
+${CMD_PREFIX} ostree --repo=repo config set 'remote "origin"'.verification-dummy-key "${DUMMYSIGN}"
+if ${CMD_PREFIX} ostree --repo=repo pull origin main 2>err.txt; then
+    assert_not_reached "pull with nosuchsystem succeeded"
+fi
+assert_file_has_content err.txt 'Requested signature type is not implemented'
+echo "ok pull failure for unknown system"
+
 repo_init
 if ${CMD_PREFIX} ostree --repo=repo remote add other --sign-verify=trustme=inline:ok http://localhost 2>err.txt; then
     assert_not_reached "remote add with invalid keytype succeeded"