support variable number of sub-second digits in `to_datetime()`

[subbyte]

The current to_datetime() function in firepit.timestamp only supports 0,3,6 sub-second digits, which is the limitation of datetime.datetime.fromisoformat().

>>> import datetime

>>> datetime.datetime.fromisoformat("2022-02-01T00:00:00")
datetime.datetime(2022, 2, 1, 0, 0)
>>> datetime.datetime.fromisoformat("2022-02-01T00:00:00.123")
datetime.datetime(2022, 2, 1, 0, 0, 0, 123000)
>>> datetime.datetime.fromisoformat("2020-06-30T19:29:59.986123")
datetime.datetime(2020, 6, 30, 19, 29, 59, 986123)

>>> datetime.datetime.fromisoformat("2022-02-01T00:00:00.1")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Invalid isoformat string: '2022-02-01T00:00:00.1'
>>> datetime.datetime.fromisoformat("2022-02-01T00:00:00.12")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Invalid isoformat string: '2022-02-01T00:00:00.12'
>>> datetime.datetime.fromisoformat("2020-06-30T19:29:59.96346")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Invalid isoformat string: '2020-06-30T19:29:59.96346'

Help needed to make it work on variable number of sub-second digits input.