datetime support in .from_dict()

Good day, assuming I have
`d = {'date_of_birth': datetime.datetime(2000)}`
and a `proto` including
`date_of_birth: Timestamp`:
When `proto.from_dict(d)` is called, it assumes the value of date_of_birth is an ISO string and does not consider that it may already be a `datetime.datetime`

I refer to your `__init__.py` L799-803:
```
elif isinstance(v, datetime):
v = datetime.fromisoformat(
value[key].replace("Z", "+00:00")
)
setattr(self, field.name, v)
```
This should check if `value[key]` is already an instance of `datetime.datetime` and assign it directly, shouldn't it?

Start in __init__.py around lines 799-803 and trace the .from_dict() handling for Timestamp fields. Verify the behavior for both an existing datetime.datetime value and an ISO-format string; done means datetime inputs are assigned without string conversion while string inputs retain their current parsing behavior.

Written by the indexing model from the issue text.