Create package with utilities for developers

[jtrh]

There are some utilities such as test factories, sample data, helpers, etc. that are not included in the library package because they are not needed for production deployments. However, they are useful during development, so we should find a way to make them easily accessible to developers.

Ref.: https://github.com/fyntex/fyndata-cl-data/blob/6d3866913c02f8e6e83889135d018ca63b015578/fd_cl_data/apps/sii_cte/tests/cl_sii_factories.py#L14-L44 Ref.: https://github.com/fyntex/fyndata-cl-data/blob/04355dbbcf40e4769b80c475dfb77e51e63ddeab/fd_cl_data/apps/sii_rcv/tests/cl_sii_factories.py#L16-L75 Ref.: https://github.com/fyntex/fyndata-cl-data/blob/6d3866913c02f8e6e83889135d018ca63b015578/fd_cl_data/apps/sii_rtc/tests/cl_sii_api_factories.py#L16-L90

CC: @glarrain