add ability to use external sharedSecret function

[mirceanis]

Context

When creating auth JWE or decrypting JWE, the current assumption is that there is access to secretKey material so that sharedKey() can be called directly.
Examples here, here and here

Problem

This prevents external crypto implementations from being used along with this library.

Solution

In a similar fashion to the Signer pattern, I'm proposing that a similar interface be used to allow the use of external crypto.

Example:

type ECDH = (theirPublicKey: Uint8Array) => Promise<Uint8Array>

with a possible implementation along the lines of:

export function createX25519ECDH(mySecretKey: Uint8Array): ECDH {
  return async (theirPublicKey: Uint8Array): Promise<Uint8Array> => {
    return sharedKey(mySecretKey, theirPublicKey)
  }
}

[awoie]

That sounds good to me, also the naming makes sense.

[uport-automation-bot]

🎉 This issue has been resolved in version 5.6.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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