Support CF templates within S3 buckets

[kangaroozal]

We have our Cloud Formation templates stored within an S3 bucket. The bucket is only read accessible by IAM members of our account.

An example fetch of the cloud formation template, using the AWS CLI, can be accomplished using the following:
aws s3 cp s3://example-bucket/cloud-formation.json ./local.json

Sfn could be enhanced to support sourcing templates from an S3 bucket using the same notation:
sfn create foo-bar --debug --file s3://example-bucket/cloud-formation.json

I'm happy to open a pull-request and attempt to implement this feature myself however I wanted to confirm this enhancement is sensible.

Cheers!

[chrisroberts]

@kangaroozal Hi! Sorry for the delay in responding to this. If you are interested in implementing this, that would be great! Running the create won't require the template to be local. Setting the TemplateURL property in the request is all that would be required. Updates may be a bit trickier as it would require fetching for diffs and plans to work. The remote template interactions would need to use the miasma lib (as the rest of the library does).

If you're not still interested, please let me know and I'll plan on getting to this feature at some point. Cheers!