(feat) add ECDH support for JWE

[ObeydKhan]

Add support for ECDH-ES Key Agreement:

Overview:
Elliptic-curve Diffie–Hellman (ECDH) is a key agreement protocol that allows two parties, each having an elliptic-curve public–private key pair, to establish a shared secret over an insecure channel. This shared secret may be directly used as a key, or to derive another key. The key, or the derived key, can then be used to encrypt subsequent communications using a symmetric-key cipher.

Steps:

• ECDH requires two sets of EC keys.
• A shared secret Z is generated using one private key and one public key. See openssl's pkey:derive
• The final Content Encryption Key (CEK) is derived using a key derivation function (KDF). It is derived from JWE header and Z.
• The derived shared key (CEK) is then used to encrypt/decrypt payload.

Some additional JWT headers set for ECDH:

• apu
• apv
• epk

References:

1. https://www.rfc-editor.org/rfc/rfc7518#appendix-C

Add support for AES-GCM-128 encryption algorithm

Related issue: #65

[ObeydKhan]

cc: @cdbattags