feat: Add PostHog telemetry provider (#9409)

Add PostHog as a telemetry provider for cloud builds so custom events
can be correlated with session recordings. Follows the same pattern as
MixpanelTelemetryProvider with dynamic import, event queuing, and
disabled events from remote config. Tree-shaken away in OSS builds.

The posthog-js package uses Apache-2.0 (verified from its LICENSE file)
but declares it as "SEE LICENSE IN LICENSE" in package.json, which
  the license checker can't parse.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9409-feat-Add-PostHog-telemetry-provider-31a6d73d3650818b8e86c772c6551099)
by [Unito](https://www.unito.io)

src/platform/telemetry/initTelemetry.ts

@@ -24,18 +24,21 @@ export async function initTelemetry(): Promise<void> {
       { TelemetryRegistry },
       { MixpanelTelemetryProvider },
       { GtmTelemetryProvider },
-      { ImpactTelemetryProvider }
+      { ImpactTelemetryProvider },
+      { PostHogTelemetryProvider }
     ] = await Promise.all([
       import('./TelemetryRegistry'),
       import('./providers/cloud/MixpanelTelemetryProvider'),
       import('./providers/cloud/GtmTelemetryProvider'),
-      import('./providers/cloud/ImpactTelemetryProvider')
+      import('./providers/cloud/ImpactTelemetryProvider'),
+      import('./providers/cloud/PostHogTelemetryProvider')
     ])
 
     const registry = new TelemetryRegistry()
     registry.registerProvider(new MixpanelTelemetryProvider())
     registry.registerProvider(new GtmTelemetryProvider())
     registry.registerProvider(new ImpactTelemetryProvider())
+    registry.registerProvider(new PostHogTelemetryProvider())
 
     setTelemetryRegistry(registry)
   })()