libotutil: Import implementation of zbase32 encoding

libotutil: Import implementation of zbase32 encoding

This will be used to implement the PGP Web Key Directory (WKD) URL
generation. This is a slightly cleaned up implementation[1] taken from
the zbase32 author's original implementation[2]. It provides a single
zbase32_encode API to convert a set of bytes to the zbase32 encoding.

I believe this should be acceptable for inclusion in ostree. The license
in the source files is BSD style while the original repo LICENSE file
claims the Creative Commons CC0 1.0 Universal license, which is public
domain.

1. https://github.com/dbnicholson/libbase32/tree/for-ostree
2. https://github.com/zooko/libbase32