Add Footer type for JSON-encoded footers

[brycx]

TODO:

• Expand tests. Especially those regarding max_len/max_keys as well
• Check error types of each method are okay. Some may still be leftover from Claims which Footer is a copy of
• Add example of using this in the high-level API/docs
• High-level API should take a Footer instance instead of Option<&[u8]>, but the lower-level APIs should still take the byte-slice. If lower-level API is used with Footer, the user can to_string().as_bytes()
• Update changelog
• Add fuzzer target (this can be combined with a fuzzer for Claims)