Rebase envoy-openssl to current release of envoy used in current release of istio

[cfilleke]

envoy-openssl hasn't had a check-in in over 2 years; there is probably much to learn from what's gone before, and yet we would like to start fresh.

• [x] archive legacy code in a branch so we can continue to refer to it.
• [ ] rebase envoyproxy/envoy-openssl/source/extensions to v 1.22
• [ ] establish contributing guidelines, document in CONTRIBUTING.md