Update `HookDispatchTxBody` with expected extension point

As we add more hook extension points, a nice belt-and-suspenders safety measure will be to add a field [here](https://github.com/hiero-ledger/hiero-consensus-node/blob/main/hapi/hedera-protobuf-java-api/src/main/proto/services/hook_dispatch.proto#L38) for the "expected extension point",
```
/**
* Details the execution of a hook.
*/
message HookExecution {
...
/**
* The extension point the target hook MUST implement.
*/
HookExtensionPoint extension_point = 3;
}

```

Start with hapi/hedera-protobuf-java-api/src/main/proto/services/hook_dispatch.proto at the referenced HookExecution definition. Confirm how HookDispatchTxBody and HookExecution are represented, then add the expected extension-point field described in the issue. Done means the schema contains the documented field and the Java protobuf API builds successfully.

Written by the indexing model from the issue text.