Add base64 for ByteBuf when serializer is human-readable

[absporl]

Depending on the is_human_readable() value for the serializer, either chooses a binary representation or base64. Implements the suggestion of https://github.com/serde-rs/bytes/issues/37 for BytesBuf.

Is this functionality something that you would merge? If so, I can look into extending it to the rest of the crate and adding some tests.