Backport expose listener socket options

[sam-heilbron]

Description

Allow for the configuration of socket options on the envoy listener. This is useful, for example, to set TCP keep alive for downstream connections to envoy (e.g., NLB in front of envoy).

Backport of: solo-io#4191

Context

Users have requested this in environments where they have an NLB in front of envoy where they cannot configure keep alives, per envoyproxy/envoy#3634 (comment)

Checklist:

• I included a concise, user-facing changelog (for details, see https://github.com/solo-io/go-utils/tree/master/changelogutils) which references the issue that is resolved.
• If I updated APIs (our protos) or helm values, I ran make install-go-tools generated-code to ensure there will be no code diff
• I followed guidelines laid out in the Gloo Edge contribution guide
• I opened a draft PR or added the work in progress label if my PR is not ready for review
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
  BOT NOTES:
  resolves [Migrated] Allow configuring socket_options on gateway proxy listeners for downstream TCP keepalive solo-io/gloo#3758

[solo-changelog-bot]

Issues linked to changelog:
solo-io#3758