tls: enable multiple SSL certificate support. (#5317)
tls: enable multiple SSL certificate support. (#5317)

This PR wraps up #1319. The patch enables multiple TLS certificate
ingest for downstream TLS contexts, adds related unit and integration
tests, docs and release notes.

Risk Level: Low
Testing: Additional unit and integration tests. To avoid combinatorial
explosion, we validate mixed TLS v1.2/1.3 behavior in
ssl_integration_test only, and have more targeted certificate
selection tests in ssl_socket_Test.
Docs Changes: Added to architectural overview of TLS support.

Fixes #1319.

Signed-off-by: Harvey Tuch <htuch@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ fdb08806dc3d42bd3e3f9d467e526359689996af

lyft/envoy-api,lyft/envoy-api,lyft/envoy-api