keystore: add support for Entrust KeyControl

[aead]

This commit adds support for the Entrust KeyControl KMS backend.

Ref: https://www.entrust.com/digital-security/key-management/keycontrol

keycontrol:
  endpoint: ""     # The KeyControl endpoint - e.g. https://keycontrol.my-org.com
  vault_id: ""     # The Vault ID            - e.g. e30497c1-bff7-4e81-beb7-fb35c4b7410c
  box_id:   ""     # The Box name or ID      - e.g. tenant-1
  # The KeyControl access credentials
  credentials:
    username: ""   # The username able to access the Vault and Box.
    password: ""   # The user password
  # The KeyControl client TLS configuration
  tls:
    ca: ""         # Path to one or multiple PEM-encoded CA certificates for verifying the KeyControl TLS certificate.