Add OtVariantBuilder

This is similar to GVariantBuilder in that it constructs variant
containers, but it writes it directly to a file descriptor rather
than keep the entier thing in memory. This is useful to create large
variants without using a lot of memory.

Closes: #1309
Approved by: cgwalters

src/libotutil/otutil.h

@@ -48,6 +48,7 @@
 #include <ot-opt-utils.h>
 #include <ot-unix-utils.h>
 #include <ot-variant-utils.h>
+#include <ot-variant-builder.h>
 #include <ot-checksum-utils.h>
 #include <ot-gpg-utils.h>
 #include <ot-checksum-instream.h>