Add support for custom headers

Purpose/Overview:
Implement the ability to add arbitrary headers to emails sent via mod-email. For a description of one particular usecase see UXPROD-2698.

Approach:
Custom headers are configured by creating entries in mod-configuration with following (required) properties:

{
      "module": "SMTP_SERVER",
      "configName": "email.headers",
      "code": "HEADER_NAME_HERE",
      "value": "HEADER_VALUE_HERE"
}

Where HEADER_NAME_HERE should be replaced with the name of your custom header, e.g. X-SES-CONFIGURATION-SET, and HEADER_VALUE_HERE should be replaced with desired value.

Values of properties module and configName should not be changed, and are required in order to be discovered by mod-email and treated as custom header configurations.

You can configure as many custom headers as you want as long as they have different code values.