[Chainstate] `SmartContract` tx wire format to indicate which Clarity version to use

[kantai]

The #2659 PR introduced Clarity versioning after the 2.1 epoch-switch, but it just applies the "default" version to all new contracts. Instead, we should support some new special top-level-only function that allows selecting the Clarity version.

[jcnelson]

Per the call today, it would be easier to add the pragma data to the TransactionPayload::SmartContract(..) variant than to add a new Clarity keyword to indicate the pragma.

[blockstack-devops]

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