Add the haystack-experimental package as a dependency

[julian-risch]

The haystack-experimental package should be added as a dependency Haystack. Users then don't need to manually install the experimental features via pip install haystack-experimental but instead automatically have them when they install Haystack.

haystack-experimental should be added here: https://github.com/deepset-ai/haystack/blob/08104e0042cff827231e30a37f53cac4d60cbc02/pyproject.toml#L42

https://github.com/deepset-ai/haystack-experimental