Allow populating event's createdAt on the send endpoint

[DavidVentura]

Related to #3156, this change allows passing createdDate on the /api/send endpoint, to specify when the event took place.
This feature is useful to import historical access logs into umami.

I've manually tested the changes with a postgres database

[vercel]

Someone is attempting to deploy a commit to the umami-software Team on Vercel.

A member of the Team first needs to authorize it.

[mikecao]

Date strings are always tricky. I think this would work better as a number, unix time in milliseconds, the same as calling Date.now().

[DavidVentura]

That was my initial implementation, but then saw yup supports validating strings as dates.

I don't have a preference to be honest