constrain contract ID types when invoking interface-member choices

[S11001001]

As of #14092, these are well-typed with respect to atContractId and choicePayload, regardless of whether the choices Mine, Another, Split were defined with ActualTemplate or inherited from an interface:

myLedger.exercise(ActualTemplate.Mine, atContractId, choicePayload)
myLedger.exercise(ActualTemplate.Another, atContractId, choicePayload)
myLedger.exercise(ActualTemplate.Split, atContractId, choicePayload)

However, this is not well-typed with respect to atContractId, though it still is with respect to choicePayload:

myLedger.exercise(SomeInterface.Split, atContractId, choicePayload)

We currently recommend that users invoke the inherited choice (a la Another above) where that is possible in order to preserve type-checking, but adding overloads causes them to be removed from inheritance.

The task here is to find a way in TypeScript to allow only atContractId either typed by SomeInterface or a template known to implement SomeInterface. Then we should consider removing inheritance.

See #14092 (comment) for context.