Design for how to improve templates acquisition experience

Our goal is to eventually redesign the templating experience in Core Tools all up (see https://github.com/Azure/azure-functions-core-tools/issues/4315). In the interim, there are potential changes we can make to improve the immediate experience - especially around acquisition.

**We should not have to redeploy a new version of Core Tools any time there is a new templates package.**

The goal of this design item is to look into what changes we can make to make it so that this template aquisition does not require its own core tools release.

The issue does not name files, tests, or an entry point. Start by reading the current template acquisition flow in Core Tools and the linked redesign issue #4315; map where the templates package is obtained and define a design that allows new template packages without a Core Tools redeployment.

Written by the indexing model from the issue text.