Allow execution of custom CloudFormation stacks

[tj]

This would allow you to define custom resources for RDS, DynamoDB etc with regular CloudFormation and/or SAM support.

The alternative is that we have more "first class" resources, mapping them out in a way which is easier to configure than for example this DynamoDB SAM example which is still quite verbose.

[polothy]

Would this include being able to supplement the current cloudformation used by Up? For example, I like everything that Up makes, but would like to add DB, IAM roles, etc.

[tj]

@mrmark yep! I haven't thought much about this yet, but that would reasonably easy to implement, and since stacks have nice input/output params it should be easy to compose them.

[mojodna]

IAM roles would be most helpful (for additional permissions beyond what's granted to the default role), either externally managed or as part of Up.