[FFI] Avoid creating duplciate ffi_types for the same structs in preparing the cif data for downcall

[ChengJin01]

This leftover issue is created to follow up the discussion at #13315 (comment) when implementing the struct support in downcall.

Technically, we should be able to return the same ffi_type for the same structs instead of creating duplicate ffi_types. e.g.

struct A {
    int a1;
    int a2;
}

struct B {
    int x;
    int y;
}

One of the challenges in this optimization is how to deal with the nested structs correctly under such circumstances. e.g.

struct A {
    int a1;
    int a2;
}

struct B {
    int x;
    struct C {  <------- struct A and C share the same ffi_type
      int c1;
      int c2;
   }
}

In addition, we probably need to ensure that the same the ffi_type for structs is shared in multithreading given these ffi_types are only released when JVM exits. That being said, we need to figure out a smart way to cover all cases to get it working to meet our anticipations.

FYI: @tajila, @pshipton

[ChengJin01]

I will need to create a couple of test case as above to go through in the existing code in debugging to better understand how everything with ffi_type is set for native signature before proposing any solution.