Plan time validation for interpolated values

[appilon]

Problem

Often times a there is a computed value that we know the general "shape" of (ex: vpc-1234) and a downstream attribute interpolates this value. That downstream value has validation to ensure it begins with say sg-. It would be helpful if we could hint to Terraform that the computed attribute has an expected format and we could have the configuration fail plantime validation.

Proposal

[paultyng]

Merging this to #61

[ghost]

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