Objectbox

The new version 0.6(.1) for the Python database comes with:
- In-memory databases (by using directory path prefix "`memory:`")
- numpy dependency fix
- Update objectbox-c to v0.21.0 with many improvements ([details](https://cpp.objectbox.io/#id-0.21.0-2024-02-13))

The new version 0.5 for the Python database brings new types:

- vector types allow efficient storage of numeric lists (and Numpy arrays), e.g. for vector embeddings in AI applications
- Date types with millisecond and nanosecond precision

More info is available at: https://objectbox.io/vector-database-flutter/

- Scalar types support 11

* add support for Apple silicon (M1)
* update to objectbox-c v0.14.0
* update flatbuffers to v2.0.0

* update objectbox-c to v0.10.0
* update flatbuffers to v1.12.0
* add transactions support
* add example
* improve makefile
* improve contribution guidelines