Expose APIs for getting pickle versions

[pksadiq]

The tests in my application fails with latest libolm as libolm updated account pickle version, I was comparing input and output pickle text, but it's failing since https://gitlab.matrix.org/matrix-org/olm/-/commit/b989db011738897e20d143417b08435b9b6955a3.

I'm not sure if the way to test these is to compare the pickle strings, but for the case, it would be nice of libolm provided APIs to get the current used pickle version for account/session (or other objects that are picklable), so that users can compare the versions and act appropriately.

fyi, I'm using libolm C API.

thanks