Add streaming base64 CBOR encoder to libutils

This implements an encoder for the Concise Binary Object Representation
that is streamed to an output as a base64 encoded bytestream.

For more details on CBOR, see [IETF RFC
7049](https://tools.ietf.org/html/rfc7049).

Signed-off-by: Curtis Millar <curtis.millar@data61.csiro.au>