AWS::CloudFormation::Stack - TemplateURL - should support any HTTP/S URL

## 2. Scope of request

This is a change to the [CreateStack](https://docs.aws.amazon.com/AWSCloudFormation/latest/APIReference/API_CreateStack.html) API, to support retrieval from locations other than S3 (for example, GitHub or a corporate artifact server).

## 3. Expected behavior

When given a publicly-accessible HTTP/S URL, CloudFormation should be able to use that URL as a template.

What this solves: currently, creating nested stacks is a two-step process. First, copy the child templates from a working directory into S3, and second, create the parent stack.

Nice-to-have: support authentication tokens for access to non-public URLs.

## 4. Suggest specific test cases

Happy path: create a stack from a simple template in the [documentation GitHub](https://github.com/awsdocs/aws-cloudformation-user-guide).

Sad path: attempt to create a stack from `http:///www.example.com/i-do-not-exist.template`.

## 6. Category (required) - Will help with tagging and be easier to find by other users to +1

5. Management (CloudTrail, Config...)