Implement ECH design doc

[nhorman]

Tracks the implementation of Encrypted Client Hello support (as defined by the design doc here. Each task below should be thought of as fitting into a sprint. When ready they will be converted into an issue for scheduling. Preliminary breakdown of future bite-size feature-branch PRs. Will likely change as review progresses.

• [ ] complete review of ECH design doc issue. This is a preliminary review. The rest of the documentation and the implementation are a separate PR that will turn into a feature branch, and will be merged incrementally.
• [ ] Merge final design and API docs into new feature branch.
• [ ] Add feature flag to enable ECH in build
• [ ] Merge ECH headers
• [ ] Merge MVP server side API
• [ ] Add support for HRR
• [ ] Merge MVP client side api
• [ ] Implement GREASE support
• [ ] Implment POC ECH enabled client
• [ ] Implement POC ECH enabled server
• [ ] Implement minimal testing of Client side api
• [ ] Implement minimal testing of Server side api

[vdukhovni]

Preliminary tweaks of the task breakdown done.

[sftcd]

@nhorman I'm not quite sure what "bite-size feature branch PRs" means, but am happy to help out as I can with getting that started and then done. (I might benefit from a voice chat with someone on that topic as you/we get to doing things.)

If each bite-size PR for a feature branch means (iterations of) adding a small chunk of the code from my implementation, then reviewing that and merging to the feature-branch, without an expectation that the feature-branch at all times does something useful, then that seems reasonable as a plan. If it means something else, then I'm less sure how doable it may be as not all useful chunks of code will be bite-sized.

That said, I do appreciate the attempt to break EPIC down into something more doable, I've been puzzled by how to get that done myself:-)

[vdukhovni]

You figured it out. The feature branch is precisely for staging smaller incremental reviews, merge what's been reviewed, and repeat. It should be created soon.