repo: new function ostree_repo_remote_get_gpg_verify_summary

It allows to specify whether GPG verification for the summary file is
enabled for a specific repository.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

doc/ostree-sections.txt

@@ -233,6 +233,7 @@ ostree_repo_remote_change
 ostree_repo_remote_list
 ostree_repo_remote_get_url
 ostree_repo_remote_get_gpg_verify
+ostree_repo_remote_get_gpg_verify_summary
 ostree_repo_remote_gpg_import
 ostree_repo_remote_fetch_summary
 ostree_repo_get_parent

doc/ostree.repo-config.xml

@@ -148,6 +148,15 @@ Boston, MA 02111-1307, USA.
         manual under GPG.</para></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><varname>gpg-verify-summary</varname></term>
+        <listitem><para>A boolean value, defaults to false.
+        Controls whether or not OSTree will check if the summary
+        is signed by a known GPG key.
+        For more information, see the <citerefentry><refentrytitle>ostree</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+        manual under GPG.</para></listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><varname>tls-permissive</varname></term>
         <listitem><para>A boolean value, defaults to false.  By