Add PostHog Cloud OAuth and self-hosted support

PostHog Cloud authenticates via CIMD OAuth (public client, PKCE)
through the region-agnostic oauth.posthog.com gateway, with an API-key
fallback. PostHog Self-Hosted is a separate provider using an API key
and an instance URL.

The shared driver discovers the data region by probing us/eu for OAuth
connections, since the gateway does not serve the data API, and pins
pagination to the resolved host.

Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>

pkg/connector/provider/builtin.go

@@ -46,6 +46,7 @@ func NewBuiltinRegistry() *Registry {
 		onePasswordRegistration(),
 		openaiRegistration(),
 		posthogRegistration(),
+		posthogSelfHostedRegistration(),
 		pagerdutyRegistration(),
 		resendRegistration(),
 		sentryRegistration(),