feat(key-manager): add method to compute a shared secret

[mirceanis]

What's here

• key-manager plugin exposes a new method that computes a shared key (Diffie Hellman) between a secret key the agent manages and a public key of another party, returning a promise that resolves to the hex encoding string of the shared key.

async keyManagerSharedKey({secretKeyRef: string, publicKey: {type: string, publicKeyHex: string}}): Promise<string>`

The result is the RAW shared secret (scalar multiplication in the case of ECC).
To use this for symmetric encryption, one would need to hash it to arrive at a shared Symmetric Key

• To do this, there is a new requirement of the AbstractKeyManagementSystem class for a new abstract method sharedKey()
• The kms-local package implements this new sharedKey method
• kms-local supports Ed25519 & X25519 keys for computing the shared key. Keys are converted to X25519 if they are Ed25519

closes #541

[codecov]

Codecov Report

Merging #555 (a861cb8) into next (0bf7b24) will increase coverage by 9.80%.
The diff coverage is 67.00%.

@@            Coverage Diff             @@
##             next     #555      +/-   ##
==========================================
+ Coverage   67.58%   77.39%   +9.80%     
==========================================
  Files          62       71       +9     
  Lines        1530     1893     +363     
  Branches      247      321      +74     
==========================================
+ Hits         1034     1465     +431     
- Misses        400      426      +26     
+ Partials       96        2      -94     

[awoie]

lgtm - just fixed some comments

[awoie]

lgtm