Port test helpers

[LDiazN]

This PR ports the code from the ooniprobe-rs repo for test helpers. Includes both the new code and the CI for generating releases.

Also closes https://github.com/ooni/ooniprobe-rs/issues/5
The following comments are copied from the original PR

---

This PR will add a new crate implementing the echo test helper and the json test helper with rust

Every test helper is implemented as a different binary that runs the test helper

Note that:

• The echo test helper uses a fixed size buffer for the request. After talking with @DecFox we concluded that this was not an issue considering the size of the payload we currently send
• The json test helper parses the request line and the headers from the socket before parsing the rest of the request with hyper, since hyper won't give you access to the request bytes nor the case-preserved headers once it's parsed

closes ooni/backend#970