Expand metadata support for enrichment

[Zethson]

Description of feature

We added lots of support to fetch metadata and the ability to enrich for Drugs (https://pertpy.readthedocs.io/en/latest/tutorials/notebooks/enrichment.html). However, for the general enrichment (currently only implemented for drugs), the metadata need to be in a specific format and stored in a specific slot (I assume).

The goal of this issue is to

• Explore and decide which other metadata we could enrich for (already available in pertpy and potentially missing)
• Store the metadata in such a way that the single enrichment function can enrich for the metadata that we deem worthy
• If this works well, we should also decouple the metadata call and how it's used in the scoring function. Currently, it implicitly uses the chembl metadata
• Update the docstrings, tutorials etc

[Zethson]

@wxicu we need at least a mention of this in the tutorials!