libotutil: Establish a place for GPG utilities

Add ot-gpg-utils.[ch] and move _ostree_gpg_error_to_gio_error() here.

src/libotutil/otutil.h

@@ -46,5 +46,6 @@
 #include <ot-variant-utils.h>
 #include <ot-spawn-utils.h>
 #include <ot-checksum-utils.h>
+#include <ot-gpg-utils.h>
 
 void ot_ptrarray_add_many (GPtrArray  *a, ...) G_GNUC_NULL_TERMINATED;