Create a KeyResolver for determining best way to access keys

Problem/Motivation

While Key is great for site builders and admins placing keys within their site, it isn't so great at automated key fetching and resolution. In order to achieve both, this issue introduces the concept of a KeyResolver, which allows other modules to intercept the entrypoint in which you obtain a key. This allows the key module to maintain its current config based structure, while allowing new methods to take hold as well.

This also allows us to do work on a 4.x version within the 1.x version without compromising BC.

Proposed resolution

Create a keyresolver class and associated helper classes and services. (see MR)