Feature request: Use YAML for all CloudFormation files

Yes. Issue #2914.

**Describe the solution you'd like**
Default support for using YAML in all generated CloudFormation stacks. Currently only JSON is supported today.

The [js-yaml](https://www.npmjs.com/package/js-yaml) package on npmjs.org is well supported, fast, and quite robust, and would work well for this scenario.

I'm happy to submit a Pull Request if supporting YAML CloudFormation Templates is a feature the Amplify team would like to include!

Note: Changing metadata, and local environment JSON files is not part of this request.

**Describe alternatives you've considered**
N/A - YAML is not supported by the Amplify CLI.

**Additional context**

YAML is:

- Smaller than JSON (in bytes)
- Easier to read
- Easier to edit
- A Superset of JSON, meaning you can parse JSON files with a YAML parser - existing JSON templates are backwards compatible
- YAML CloudFormation templates support short form Intrinsic Functions, such as `!Sub`, `!Ref` and `!Join` - further reducing template size, and improving readability

JSON is:

- Significantly larger on-disk for the same data compared to YAML
- Arguably Harder to read and edit than YAML
- JSON is Valid YAML (but YAML is not valid JSON)

Strictly as matter of preference, YAML is arguably more developer-friendly than, and preferred over JSON.

As an example, using the CloudFormation template generated by [this schema](https://github.com/rapticore/amplify-stack-size-bug/blob/master/schema.graphql), the YAML CloudFormation stack is 271KB versus 820KB. YAML version created using `cfn-flip`.

```
-rw-r--r-- 1 rbowen staff 820K Dec 3 01:16 cloudformation-template.json
-rw-r--r-- 1 rbowen staff 271K Dec 3 01:31 cloudformation-template.yaml
```

Start by tracing where the Amplify CLI generates CloudFormation stacks and how it currently serializes JSON templates. Evaluate js-yaml support across those generated stacks, while keeping metadata and local environment JSON files unchanged; done means generated templates support YAML without breaking existing JSON templates.

Written by the indexing model from the issue text.