feat(DelegatedHttpGateway): allow top-level config setting/sharing

[gunnar-solo]

Description

This PR expands on the functionality of DelegatedHttpGateway and MatchableHttpGateway. It allows for anscestry-level setting of TLS and HCM configuration. Phrased differently: configuration can now be set for a single DelegatedHttpGateway that will be propagated to all MatchableHttpGateways. Setting the preventChildOverrides flag specifies that parent config is to be prioritized over child. For example:

• preventChildOverrides=false | {"parent":{"foo":"bar"}, "child":{"foo":"baz"}} --> {"foo":"baz"}
• preventChildOverrides=true | {"parent":{"foo":"bar"}, "child":{"foo":"baz"}} --> {"foo":"bar"}

Context

This feature is intended to be useful to a multi-team gloo management environment. Picture a top-level, well-informed team managing a single DelegatedHttpGateway with a bunch of useful defaults. Then, individual teams/microservices manage their own MatchableHttpGateway(s)

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 -B 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

[gunnar-solo]

/skip-changelog

re: backported feature