Support firstdata_e4 gateway API v13, add HMAC

This PR adds support for FDe4 gateway API v13.
The primary additional feature is HMAC based API security introduced in v12.
FDe4 Merchants will need to pass in hmac_key_id and hmac_key, retrievable from the FDe4 admin portal as described here:
https://firstdata.zendesk.com/entries/22069302-api-security-hmac-hash