api: add socket buffer limit field in ClientTrafficPolicy and BackendTrafficPolicy

[aoledk]

What this PR does / why we need it:

Allow user to configure socket buffer limit for both incoming socket and outgoing socket to control memory consumed by Envoy, especially useful for user case of long-lived L4 proxy for massive connections.

It's different from existing connection buffer limit api which is used to control user space memory consumption, and this new api is used to control kernel memory consumption.

Which issue(s) this PR fixes:

Fixes #2670