Panic if asked to generate multi-argument invokes

Presently, Java code generation does not support the `MultiArgumentInputs` field
on invokes. When set, this flag instructs the generation of functions that take
multiple independent arguments instead of a single object containing all
arguments. For example, instead of:

```java
Class.function(FunctionArgs.build().setA(a).setB(b).build());
```

we might generate:

```java
Class.function(a, b);
```

Since it is not supported, the correct behaviour is to panic and abort code
generation, as we do in other languages with this limitation (e.g. Python).
Currently, however, Java does not, proceeding instead to generate incorrect
code. This change fixes that, panicking if `MultiArgumentInputs` is true. This
is technically breaking, but a bug fix (and in theory, any SDKs we have
generated would already be panicking for Python, so the feeling is that this
likely not a highly used feature).

Fixes #1598

pkg/codegen/java/gen_program_expressions.go

@@ -322,6 +322,12 @@ func (g *generator) GenFunctionCallExpression(w io.Writer, expr *model.FunctionC
 		// Assuming the existence of the following helper method
 		g.Fgenf(w, "computeFileBase64Sha256(%v)", expr.Args[0])
 	case pcl.Invoke:
+		if expr.Signature.MultiArgumentInputs {
+			err := fmt.Errorf("Java program-gen does not implement MultiArgumentInputs for function '%v'",
+				expr.Args[0])
+			panic(err)
+		}
+
 		fullyQualifiedType, funcName := g.functionImportDef(expr.Args[0])
 		if !g.emittedTypeImportSymbols.Has(fullyQualifiedType) {
 			// the fully qualified name isn't emitted