[SkusSDK] - Improve Rust-C++ FFI

[Brandon-T]

Description:

• Current Rust and C++ communication is rough. It requires generating callbacks on the C++ side and passing them to Rust and hoping that the caller passes the correct arguments. Otherwise it will crash.
• Current Rust and C++ communication requires creating a callback-state holder that holds the callback, and a bunch of callback signatures:

type RefreshOrderCallbackState;
type RefreshOrderCallback = crate::RefreshOrderCallback;
type FetchOrderCredentialsCallbackState;
type FetchOrderCredentialsCallback = crate::FetchOrderCredentialsCallback;
type PrepareCredentialsPresentationCallbackState;
type PrepareCredentialsPresentationCallback = crate::PrepareCredentialsPresentationCallback;
type CredentialSummaryCallbackState;
type CredentialSummaryCallback = crate::CredentialSummaryCallback;
type SubmitReceiptCallbackState;
type SubmitReceiptCallback = crate::SubmitReceiptCallback;
type CreateOrderFromReceiptCallbackState;
type CreateOrderFromReceiptCallback = crate::CreateOrderFromReceiptCallback;

where create::XXXX is also defined in C++ via a shim.h file.

• Current Rust and C++ communication also requires Unsafe code and "extern C" style functions where the first argument is a pointer to an instance allocated on the heap, and the second argument is the function to be called, and it's called via a trampoline:

#[allow(improper_ctypes_definitions)]
#[repr(transparent)]
pub struct RefreshOrderCallback(
    pub  extern "C" fn(
        callback_state: *mut ffi::RefreshOrderCallbackState,
        result: ffi::SkusResult,
        order: &str,
    ),
);

unsafe impl ExternType for RefreshOrderCallback {
    type Id = type_id!("skus::RefreshOrderCallback");
    type Kind = cxx::kind::Trivial;
}

This is done for every single C++ function that Skus Rust code needs to call. It's tedious, and error prone, and can easily lead to Undefined Behaviour if proper care is not taken to ensure the right signatures and arguments are passed on the heap.