Adding sidecar, serviceEntry to whitelist egress

General MR

Summary

This MR introduces a Sidecar and a set of ServiceEntries for MatterMost when istio.enabled: true and istio.hardened.enabled: true. This is in support of big-bang&160.

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Linked Issue

For issue

Upgrade Notices

A Sidecar resource has been added to the MatterMost namespace that disallows egress to endpoints that are not part of the Istio service registry (a.k.a REGISTRY_ONLY). The outboundTrafficPolicy.mode in the Sidecar can be configured, however, to be something other than REGISTRY_ONLY if desired by setting istio.hardened.outboundTrafficPolicyMode. This provides a redundant layer of network security in addition to NetworkPolicies. This Sidecar is disabled by default but can be enabled by setting istio.enabled: true and istio.hardened.enabled: true.

Additionally, custom ServiceEntries can be created by populating the istio.hardened.customServiceEntries list.