feat: Allow different environment variables for multiple Diffie-Hellman keys

[dbellinghoven]

Description of changes:

• Allow the user to set different DH keys for different sinks. For example, if I wish to use two sinks, both of which should be encrypted with different keys, I can set the DH private key for the first sink with the environment variable DH_PRIV_KEY_1 and I can set the other private key with the DH_PRIV_KEY_2 environment variable. Then I would set these variables in my config.hcl file:

  auto_auth {
  method "aws" {
      mount_path = "auth/aws"
      config = {
          type    = "iam"
          role    = "foobar"
          secrets = {
                              registry-1.example.com = "secret/docker/registry1"
                              registry-2.example.com = "secret/docker/registry2"
                      }
      }
  }
  
  sink "file" {
      dh_type = "curve25519"
              dh_path = "/tmp/dh-pub-1.json"
      config  = {
          path = "/tmp/file-foo"
          dh_priv_env = "DH_PRIV_KEY_1"
      }
  }
  
  sink "file" {
      dh_type = "curve25519"
              dh_path = "/tmp/dh-pub-2.json"
      config  = {
          path = "/tmp/file-foo"
          dh_priv_env = "DH_PRIV_KEY_2"
      }
  }
  }

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.