Surveys api/surveys API should be a POST and have a distinct id param

[marandaneto]

context https://posthog.slack.com/archives/C07QD3LT8U9/p1738359117136199

The surveys API returns all active surveys for an apiKey, including already answered surveys since the API does not know who is requesting it (there's no distinct_id query parameter or something).
The JS SDK has to filter out already answered surveys.

The current API is a GET https://us.i.posthog.com/api/surveys/?token=apiKey&ip=1&_=12345678&ver=1.211.2

• change endpoint to POST (keep the GET for backcompatibility)
• send distinct ID over the wire
• just return the surveys that match and stop filtering on client-side

[marandaneto]

Closing this in favor of PostHog/posthog-js#1652
There'll be many more benefits with surveys being a static file + caching, which means some code dupe on the client side, but it's worth it.