add c_hash, s_hash, update at_hash

[panva]

Its value is the base64url encoding of the left-most half of the hash of the octets of the ASCII representation of the code value, where the hash algorithm used is the hash algorithm used in the alg Header Parameter of the ID Token's JOSE Header. For instance, if the alg is HS512, hash the code value with SHA-512, then take the left-most 256 bits and base64url encode them.

added c_hash (core), s_hash (fapi part 2) and updated at_hash not to mention SHA256 since it's only valid for *256 signed tokens.

[sebadoom]

Merged, thank you for your contribution.