This extension will be used to measure the latency impact of potentially
sending a post-quantum key share by default. At this time it's purely
measuring the impact of the client sending the key share, not the server
replying with a ciphertext.

We could use the existing padding extension for this but that extension
doesn't allow the server to echo it, so we would need a different
extension in the future anyway. Thus we just create one now.

We can assume that modern clients will be using TLS 1.3 by the time that
PQ key-exchange is established and thus the key share will be sent in
all ClientHello messages. However, since TLS 1.3 isn't quite here yet,
this extension is also sent for TLS 1.0–1.2 ClientHellos. The latency
impact should be the same either way.

Change-Id: Ie4a17551f6589b28505797e8c54cddbe3338dfe5
Reviewed-on: https://boringssl-review.googlesource.com/24585
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: David Benjamin <davidben@google.com>