feat: allocate FFI buffer 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 caller implementation can be found in the Java repository. All tests pass on my machine.

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