Capture EPCs for Drug Concepts

We should consider grabbing [established pharmacologic classes (EPC)](https://www.fda.gov/industry/structured-product-labeling-resources/pharmacologic-class#:~:text=An%20FDA%20%E2%80%9CEstablished%20Pharmacologic%20Class,scientifically%20valid%20and%20clinically%20meaningful.) when available for drug concept groups. This information would have clinical benefits for downstream applications of Thera-py, such as in DGIdb where this EPC could help inform clinical relevance of interaction data.

This is available from sources like DailyMed. An example from https://rxnav.nlm.nih.gov/REST/rxclass/class/byDrugName.json?drugName=imatinib&relaSource=DAILYMED&relas=has_epc ->

```
{
"rxclassDrugInfoList": {
"rxclassDrugInfo": [
{
"minConcept": {
"rxcui": "282388",
"name": "imatinib",
"tty": "IN"
},
"rxclassMinConceptItem": {
"classId": "N0000175605",
"className": "Kinase Inhibitor",
"classType": "EPC"
},
"rela": "has_epc",
"relaSource": "DAILYMED"
}
]
}
}
```

Use the RxNav/DailyMed response in the issue as the reference input. Trace the repository’s existing drug concept-group normalization path, then determine how available EPC data should be captured for downstream consumers. Done means EPC information is represented for applicable drug concept groups and verified against the provided example.

Written by the indexing model from the issue text.