Handle spec version is a much better way

[xntrik]

Currently this is tied to the version of hcltm itself, and doesn't do anything except for output in weird ways.

The spec itself should be a an interface, with different versions implementing the interfaces, allowing for handling of different versions.

[xntrik]

This may not be the final work on this ticket, but, I'm starting to construct a constraint system that will allow for degradation warnings etc.

For instance, you'll be able to define a struct that meets a hcltmConstraint interface. This includes:

• version constraint string (using hashicorp's go-version for comparisons)
• a threat model check function, allows each new constraint to actually parse a TM to validate whether they need to worry about the upcoming constraint
• message - to be returned if the above two are true.