feat: allocate FFI output from Rust

[tbrezot]

In order to avoid allocating an upper bound on the amount of memory needed in the FFI caller, I propose the following workaround:

1. the caller allocates the size of a pointer
2. Rust sets the value of this pointer to a pointer to the allocated buffer
3. Rust forgets about the buffer (avoids freeing the memory pointer to by the returned pointer)
4. the caller unwraps the pointer and copies the bytes to a proper structure
5. the caller free the pointer

This MR applies this method to the FFI API of Findex upsert. The Rust implementation can be found in the Rust repository. All tests pass on my machine.

I need particular proofreading on the free step (5) and potential incompatibilities with a Flutter/C implementation.

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[ThibaudDauce]

I just checked on Dart, it seems hard to do the same thing… I'll continue to investigate.

[ThibaudDauce]

I just checked on Dart, it seems hard to do the same thing… I'll continue to investigate.

Something like this may work:

      final bytes = Pointer.fromAddress(output.value).cast<Uint8>();
      malloc.free(bytes);

      return deserializeSearchResults(
        bytes.asTypedList(outputLengthPointer.value),
      );