Add PostHog source UI with region select

Show a US/EU region dropdown for the PostHog Cloud API-key form and
an instance-URL field for Self-Hosted, map both to the connector inputs,
and register the PostHog logo for the self-hosted provider.

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

packages/ui/src/Atoms/ThirdParties/ThirdPartyLogo.tsx

@@ -78,6 +78,7 @@ const thirdParties: Record<string, FC<ComponentProps<"svg">>> = {
   OPENAI: OpenAI,
   PAGERDUTY: PagerDuty,
   POSTHOG: PostHog,
+  POSTHOG_SELF_HOSTED: PostHog,
   RESEND: Resend,
   SENTRY: Sentry,
   SLACK: Slack,