feat(external-call): Add proto definitions and data types

[trusch]

Summary

This PR adds the foundational proto definitions and Scala data types for the external call feature. It defines how external call results are represented in transactions and the language AST.

Changes

Proto Definitions

daml_lf2.proto

enum BuiltinFunction {
  // ... existing values ...
  BUILTIN_EXTERNAL_CALL = 74;
}

transaction.proto

message ExternalCallResult {
  string extension_id = 1;
  string function_id = 2;
  bytes config = 3;
  bytes input = 4;
  bytes output = 5;
}

message Node.Exercise {
  // ... existing fields ...
  repeated ExternalCallResult external_call_results = 18;
}

Scala Data Types

Node.scala

• Added externalCallResults: Seq[ExternalCallResult] to Exercise node
• Added ExternalCallResult case class

Ast.scala

• Added BEExternalCall to BuiltinFunction sealed trait

LanguageFeatureGen.scala

• Added feature flag for external calls

Transaction.scala

• Updated transaction traversal to include external call results

SerializationVersion.scala

• Added support for serializing external call results

Validation

Typing.scala

• Added type checking for BEExternalCall builtin

Decoding

DecodeV2.scala

• Added decoder for BUILTIN_EXTERNAL_CALL

Testing

• TransactionSpec.scala - Transaction with external calls
• ValueGenerators.scala - Generators for external call results

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[paulbrauner-da]

Overall looks good but please address comments.

[paulbrauner-da]

config_hash, input_hex, and output_hex should have type bytes, not string. The daml language doesn't support strings, that's why we use an hex encoding in LF. (We may introduce proper support for bytes in LF one day.) But in the transaction, it is a waste of space to represent them as strings.

[paulbrauner-da]

This not needed because repeated is ordered.

[paulbrauner-da]

Same comment as in the protobuf definition: configHash, inputHex and outputHex should be daml.lf.data.Bytes. callIndex is not necessary.

[paulbrauner-da]

LF (the language that the daml compiler produces and that the engine consumes) is separate from the language of the transactions produced by the engine, even if they tend to refer to them as "LF transactions". For that reason, we tend to introduce LF changes and transaction changes in separate PRs. I suggest you split this PR into a transaction one and an LF one.

When you modify LF you also need to:

• Modify LF syntax test parser in com.digitalasset.daml.lf.testing.parser and its spec.
• Add a trivial test for typing in TypingSpec (which uses the parser above)
• Add it to Builtin_2.dev_.lf