Allow `timestamp` to be set in `posthog-server` `identify` calls

[luke-belton]

Problem Statement

We recently added support for custom timestamp properties in capture calls - customers would also like this for identify calls.

Context: https://posthoghelp.zendesk.com/agent/tickets/38636

As a workaround, you can just send $identify events manually via the capture method.

Solution Brainstorm

No response

[marandaneto]

@dustinbyrne something to consider