BUG: PeriodIndex.asfreq silently converts offset to period

[natmokval]

xref #52064

PeriodIndex.asfreq silently converts for offsets such as offsets.MonthBegin(), offsets.BusinessMonthEnd(), etc. (with no attribute '_period_dtype_code') frequency to period frequency (in this case 'M').

Reproducible Example:

>>> import pandas as pd
>>> from pandas.tseries import offsets
>>>
>>> index = pd.PeriodIndex(["2020-01-01", "2021-01-01"], freq="Q")
>>> index.asfreq(freq=offsets.MonthBegin(1))
PeriodIndex(['2020-03', '2021-03'], dtype='period[M]')
>>>
>>> index.asfreq(freq=offsets.BusinessMonthEnd(1)) 
PeriodIndex(['2020-03', '2021-03'], dtype='period[M]')

the correct behaviour would be raising an Error:

>>> index.asfreq(freq=offsets.MonthBegin(1))
ValueError: <MonthBegin> is not supported as period frequency
>>>
>>> index.asfreq(freq=offsets.BusinessMonthEnd(1))
ValueError: <BusinessMonthEnd> is not supported as period frequency

corrected the definition of PeriodIndex.asfreqand added tests.

[natmokval]

@MarcoGorelli, could you please take a look at this PR?

[MarcoGorelli]

I'd like to think that the suggestion in #56945 (review) (i.e. raise if the return value of to_offset(..., is_period=True) doesn't have a _period_dtype_code attribute, could solve this too?

[natmokval]

closed by #56945