ENH: fix arrow roundtrip for ExtensionDtypes in absence of pandas metadata
An oversight in our Arrow Extension Types for period and interval to also define the conversion to the equivalent pandas dtype. This ensures that the arrow->pandas conversion also works if there is no "pandas_metadata" in the schema's metadata, eg when this metadata was lost during some operations (up to now, the pandas ExtensionDtype was reconstructed based on the string name in the stored metadata).

See https://arrow.apache.org/docs/python/extending_types.html#conversion-to-pandas
lgtm. do we need a whatsnew note? A whatsnew note for it might be a bit "obscure", but can certainly add one if preferred