New package for secrets retrieval from AWS KMS

[Robert-W]

Includes test, documentation, and has been prettified.

Usage is simple but requires AWS authentication.

const secrets = require('@asymmetrik/fhir-secrets');

secrets.configure({ region: 'us-east-2' })
  .decrypt({ CiphertextBlob: '' })
  .then(plaintext => doStuffWithSecret(plaintext))
  .catch(err => {
    // Unable to obtain secrets
  });