navajo: auto-detect whether base64 values are padded

[chanced]

Check if the value ends in "=". If so, use base64::engine::general_purpose::URL_SAFE rather than URL_SAFE_NO_PAD

Code needing to be updated:

• b64 mod
• jose::Header::try_from(value: &[u8])
• jose::Jws<Vec<u8>>::from_str(jws: &str)