Goal

Emit a "handshake event", allowing customers to observe multiple pieces of information about the handshake.

Why

Currently customers have to call large numbers of individual "getter" APIs to gather information about the handshake. We expect that with this event structure will eventually be used to aggregate metrics for e.g. emission to CloudWatch.

How

The handshake is emitted to the application through a callback.

Callouts

1. all of these APIs are currently unstable, and are expected to remain so for the forseeable future. Accordingly everything in this API is a "two way door".
2. s2n_event_handshake vs s2n_handshake_event? I went with the former because it felt more neatly namespaced.
3. I wonder whether we want to have the handshake event emitted by value?

This is adding new code in the hotpath, but it has a minimal impact

handshake-secp256r1/s2n-tls
                        time:   [1.1705 ms 1.1711 ms 1.1718 ms]
                        change: [−0.1650% +0.6024% +1.3401%] (p = 0.08 > 0.05)

Testing

Unit tests, and self-talk tests in the rust bindings.

The tests for this cover a large API surface area, which is tedious to cover in C. Therefore I deliberately have most of the testing in Rust.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.