ENH: `.to_timedelta` method for `PeriodIndex` and `Period` objects 
#### Is your feature request related to a problem?

I oftentimes find myself needing the duration of a `Period` or `PeriodIndex`, which I calculate with 
```
timedelta = (p+1).start_time - p.start_time
```
It seems useful and commonly used enough to be included as a standard method.

#### Describe the solution you'd like

Additional `.to_timedelta` method, just as there exists a `.to_timestamp` method on these objects.

#### API breaking implications

None afaics.

#### Describe alternatives you've considered

Use a `.duration` attribute instead, which does the conversion from timedelta to seconds `((p+1).start_time - p.start_time).total_seconds()`, but that makes little sense.

#### Additional context

Sample:

```python
pi = pd.period_range('2019', periods=3, freq='Y')
pi
PeriodIndex(['2019', '2020', '2021'], dtype='period[A-DEC]', freq='A-DEC')

tdi = ((pi+1).start_time - pi.start_time) #now
tdi = pi.to_timedelta() #as suggested
tdi
TimedeltaIndex(['365 days', '366 days', '365 days'], dtype='timedelta64[ns]', freq=None)

# finding the middle of a period
mid_ts = pi.start_time + 0.5*((pi+1).start_time - pi.start_time) #now
mid_ts = pi.start_time + 0.5*pi.to_timedelta() #as suggested
mid_ts
DatetimeIndex(['2019-07-02 12:00:00', '2020-07-02 00:00:00', '2021-07-02 12:00:00'],
              dtype='datetime64[ns]', freq='8772H')
```
