Helpers for constructing JNI signatures

To construct strings such as `"I(I)"` we could add some helper code that could construct them from Dart function types.

```dart
String typeToJavaSignature() {
final splitOnArrow = T.toString().split(' => ');
final returnType = splitOnArrow[1];
final arguments = splitOnArrow[0];
final argumentsTrimmed = arguments.substring(1, arguments.length - 1);
final argumentTypes = argumentsTrimmed.split(', ');
final javaReturn = dartTypeToJavaSignatureType[returnType]!;
final javaArguments =
argumentTypes.map((e) => dartTypeToJavaSignatureType[e]!).join(',');
return '($javaArguments)$javaReturn';
}
```

```dart
final javaSignature = typeToJavaSignature();
```

The is probably not general enough, but maybe it would be a nice addition to the jni interface over having users having to construct the strings themselves.