Enable configuration of sub-second precision of timestamps

[GrahamCampbell]

This PR enables the configuration of the sub-second precision in serialized timestamps. None of these changes are breaking because either the changes are made to classes marked as internal or a new constructor added to a final class with all parameters optional, and the default precision remains unchanged.

Example usage, serializing a cloudevent timestamp including the milliseconds but not the microseconds:

$cloudEvent = (new CloudEvents\V1\CloudEventImmutable('a', 'b', 'c'))
    ->withTime(new DateTimeImmutable('2023-01-01T01:23:45.567890Z'));

$serializer = new CloudEvents\Serializers\JsonSerializer(
    new CloudEvents\Serializers\Normalizers\V1\Normalizer([
        'subsecondPrecision' => 3,
    ]),
);

// {"specversion":"1.0","id":"a","source":"b","type":"c","time":"2023-01-01T01:23:45.567Z"}
echo $serializer->serializeStructured($cloudEvent);

Note that on the deserialisation side of the house, we already can handle sub-second timestamps of any precision, truncating down to at most 6 digits, as supported by PHP, without any configuration.