jextract should add helpers for accessing struct fields inside an array of structs

It is common to allocate an array of struct, and then set various fields in all the elements of the struct array.

Currently, jextract generates static getter/setters which work great when operating on a single struct - but fail to scale in the array case - example:

MemorySegment add_inputs = s.allocateArray(TF_Output.$LAYOUT(), 2);
TF_Output.oper$set(add_inputs, one);
TF_Output.index$set(add_inputs, 0);
MemorySegment snd = add_inputs.asSlice(TF_Output.$LAYOUT().byteSize());
TF_Output.oper$set(snd, two);
TF_Output.index$set(snd, 0);

Since the getter/setters take only a memory segment, we have no other way to do the above, other than slicing the segment and get a segment for the second element.

If jextract added also array accessors, the above could be greatly simplified as:

TF_Output.oper$setArray(add_inputs, 0, one);
TF_Output.index$setArray(add_inputs, 0, 0);
TF_Output.oper$setArray(add_inputs, 1, two);
TF_Output.index$setArray(add_inputs, 1, 0);

Now, the new getter/setter also take an additional parameter - the index of the struct element to be set/get.