Add function to de-/encrypt data asymmetrically

[dominic22]

Add two utils function to the is-shared-modules with following features:

asymEncrypt:

• Should be able to encrypt data using X25519 key stored in a identity document (function does not need to query the key it will be passed into as parameter)

asymDecrypt:

• Should be able to decrypt data using the corresponding X25519 private key

[dominic22]

see this https://app.zenhub.com/workspaces/cloud-services-6257fb8bd79c33001e24c112/issues/iotaledger/cloud-services/83