Format datetime field with millisecond precision

Since `datetime.datetime.strftime` doesn't have an easy way to convert a datetime with microsecond precision to millisecond precision this also leaks into Marshmallow.
I have come up with
```
from marshmallow import fields
from datetime import datetime

from marshmallow import Schema
class Blah(Schema):
date_time = fields.Function(
lambda dictionary: dictionary["date_time"].strftime("%FT%T.%f")[:-3] + "Z"
)
x = datetime.utcnow()
print(f"{x=}") # prints x=datetime.datetime(2022, 8, 9, 16, 38, 44, 165105)
Blah().dumps({"date_time":x}) # prints '{"date_time": "2022-08-09T16:38:44.165Z"}'
```
Is there a cleaner way to do this?

I also dislike the fact that this is now linked to the input type, if I now use a class to dump the data from instead of a dict I will have to change the mapping to be
```
class Blah(Schema):
date_time = fields.Function(
lambda some_instance: some_instance.date_time.strftime("%FT%T.%f")[:-3] + "Z"
)
```
I feel I'm missing something

The issue names no repository files or tests; begin with fields.Function and Schema and reproduce the shown Blah().dumps example. Define done as a supported way to serialize datetime values with millisecond precision without coupling the mapping to dict versus object input, with tests covering both inputs.

Written by the indexing model from the issue text.