Include the HTTP gateway in Holonix

[ThetaSinner]

To developers building against the HTTP gateway, we should optionally include it in Holonix.

AC:

• There is a new binary available in Holonix for the HTTP gateway
• Documentation describes how to include it but it is not provided in the default flake template
• Only available at 0.4 for now
• If the gateway is not requested by a consuming flake, then it does not need to be built