Index name defined in DataFrameModel is discarded when creating DataFrameSchema

[crsren]

Describe the bug
When creating a schema from a DataFrameModel, the name of the index is discarded.

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of pandera. (0.18.0)
• (optional) I have confirmed this bug exists on the master branch of pandera.

Code Sample

class Pinapple(pa.DataFrameModel):
    id: Index[int]
    name: Series[str]
    
pinapple_schema = Pinapple.to_schema()

Expected behavior

I'm expecting pinapple_schema to have the attribute index=<Schema Index(name=id, type=DataType(string[pyarrow]))>. While the type is correct, the name instead is just None.