Allow additional headers to be part of the signature payload

[jacobweinstock]

Hello, thanks for the great tool!

I am interested in adding a new feature. The feature would allow for concatenating additional header values to the request body in order to create the signature payload for the payload-hmac-sha1, payload-hmac-sha256, payload-hmac-sha512 match rules. This feature would allow for greater flexibility in the composition of the signature payload.

This is especially useful when implementing relay prevention. A timestamp header can be included in the signature payload and then the execute-command can confidently use the timestamp header to validate if requests have been sent recently and reduce the risk of message replays.