Feature request: Add diagnostics helper functions

[golles]

Would it be possible to include get_diagnostics_for_config_entry and _get_diagnostics_for_config_entry? (and maybe also get_diagnostics_for_device and _get_diagnostics_for_device) from https://github.com/home-assistant/core/blob/dev/tests/components/diagnostics/__init__.py

In my component, I've added a test for diagnostics and had to copy the 2 mentioned functions from HA into my project (but maybe there is a better way that I'm not aware of?)