Add interface_id in Ledger API Exercise Command

In case of exercise-interface, we abuse the field templateId of the Exercise Command to be actually an interfaceId.

We will add a new file interface_Id to the exercise command

// Exercise a choice on an existing contract.
message ExerciseCommand {
  // The template or the interface where the choice is defined.
  // One and only one of template_id interface_id is required.
  Identifier template_id = 1;
  Identifier interface_id = 5;

  // The ID of the contract the client wants to exercise upon.
  // Must be a valid LedgerString (as described in ``value.proto``).
  // Required
  string contract_id = 2;

  // The name of the choice the client wants to exercise.
  // Must be a valid NameString (as described in ``value.proto``)
  // Required
  string choice = 3;

  // The argument for this choice.
  // Required
  Value choice_argument = 4;
}

[S11001001]

using a field name called template_id to specify an interface_id is very confusion.

There’s a smaller change to deal with the naming problem without breaking how commands are built in current codegen and JSON API.

1. add a contract_type_id to ExerciseCommand, or whatever you care to call the union of template IDs and interface IDs (I have generally called it “contract type ID” but have not seen any other opinions on this)
2. give it exactly the same semantics as template_id does now; command resolves to contract_type_id or template_id, preferring the former
3. deprecate template_id

[S11001001]

it will not be possible to abuse the templateId to specify interface ID.

Then, maybe this should be added to exercise-by-key and create-and-exercise first? At least for those commands, it lets you express something that is impossible, currently. Adding it to by-id will just cause regressions in clients like JSON API.

The current behavior doesn’t seem like “abuse” at all. It is really just a case of a field that is now misnamed; rename it contract_type_id and suddenly it isn’t “abuse”.