Rewrite echoth and http return JSON header test helper

[hellais]

Currently the echoth and return JSON headers test helpers are written in twisted and are very annoying to maintain and run.

We should consider rewriting these as small microservices that we can more easily deploy. Either a simple rust or golang service would be good, that we can ship as a binary wherever we want.

This is related to: ooni/devops#224

See original source code:

• HTTPReturnJSONHeadersHelper: https://github.com/ooni/backend/blob/v1.3.6/oonib/testhelpers/http_helpers.py#L140
• TCPEchoHelper: https://github.com/ooni/backend/blob/v1.3.6/oonib/testhelpers/tcp_helpers.py#L14