Replace CLI walkthroughs with a "single source of truth" config file.

**Is your feature request related to a problem? Please describe.**
In my opinion, lengthy and complex CLI walkthroughs lead to poor dev UX.

More specifically, they tend to obscure documented bugs with copy-pastes of walkthroughs, which only get more and more outdated. In addition, as of now, they don't 100% consistently keep the previously selected value/result (when using `amplify update`) leaving us vulnerable to overwriting what we've already configured. There are likely more cons that I haven't described here.

**Describe the solution you'd like**
I'd love to use a single config file or perhaps a directory of specifically named config files (e.g. `analytics.json`). Something very similar to the already existing`backend-config.json`, but obviously with more detail and capabilities, would be ideal.

In addition, it'd be nice if the amplify project could rely heavily on this config file(s), to the point where I could look to it as the "single source of truth" for my amplify project. Currently, there are a multitude of locations I have to look in when I debug (e.g. `backend-config.json`, `amplify-meta.json`, `aws-exports.json`, `team-provider-info.json`, cloudformation files, etc.) and while it may be impossible to eliminate the need to check *all* of these locations, it'd be nice if logic such as the `dependsOn` key/value in `backend-config.json` and `amplify-meta.json` could be consolidated into one file.

**Additional context**
I've been forced into several occasions where I've had to delete either the entire amplify project or remove categories because of bugs I've run into during update walkthroughs. This post is largely due to these incidents.

I mean no offense to the amplify team, *I love* what I've been able to build with Amplify so far and I appreciate all their hard work.