[Spike] Explore iterable gateway configuration

Motivation

Currently the operatorless istio sandbox has gateway configuration split out into separate template directories. This leads to some measure of repeated code. In order to DRY this up, we should explore using an iterable object to create gateway HelmReleases and its dependencies

Acceptance Criteria

• A detailed account of the trade-offs between approaches must be commented to this issue
• A branch with an example configuration must be linked in this issue
• A discussion with SM team and anchors about which approach to go with must be had and decision documented on this issue