Add a setPostHogUserID method

[alpennec]

Hello,

Now that you support PostHog, can you update the SDK to add a method to set the UserID instead of having to use the reserved attribute $posthogUserId ourselves?

https://www.revenuecat.com/docs/integrations/third-party-integrations/posthog

Thanks,
Axel

[RCGitBot]

👀 We've just linked this issue to our internal tracker and notified the team. Thank you for reporting, we're checking this out!

[HaleyRevcat]

Hi, I have passed this feedback along to the team.

[codykerns]

Hey @alpennec! This is implemented in #4679, and should be included in the next release 😃