[Lib] Implement token verification builtins

[anakrish]

• io.jwt.decode
• io.jwt.decode_verify
• io.jwt.verify_es256
• io.jwt.verify_es384
• io.jwt.verify_es512
• io.jwt.verify_hs256
• io.jwt.verify_hs384
• io.jwt.verify_hs512
• io.jwt.verify_ps256
• io.jwt.verify_ps384
• io.jwt.verify_ps512
• io.jwt.verify_rs256
• io.jwt.verify_rs384
• io.jwt.verify_rs512

Ensure that cargo test -r --test opa -- jwtbuiltins jwtdecodeverify jwtverifyhs256 jwtverifyhs384 jwtverifyhs512 jwtverifyrsa passes.