Add support for offshore outages

[fboerman]

querying offshore outages can be done by below snippit but then the parser fails

from secret import api_key
from entsoe import EntsoePandasClient
import pandas as pd
client = EntsoePandasClient(api_key)
df = client._query_unavailability(country_code='DE_TENNET', start=pd.Timestamp('2024-09-18', tz='europe/amsterdam'), end=pd.Timestamp('2024-09-20', tz='europe/amsterdam'), doctype='A79')

the outage parser should also handle curvetype better, so perhaps rewrite this as well to use the generic parser