feat(tracing): provider-agnostic exporters with first-class PostHog support (#972)

* feat(tracing): add provider-agnostic exporters with first-class PostHog support

* chore(config): regenerate full reference rendered config for exporters

* refactor(tracing): drop posthog exporter 'enabled' flag per review

config/plano_config_schema.yaml

@@ -447,6 +447,28 @@ properties:
             additionalProperties:
               type: string
         additionalProperties: false
+      exporters:
+        type: array
+        items:
+          oneOf:
+            - type: object
+              properties:
+                type:
+                  type: string
+                  const: posthog
+                url:
+                  type: string
+                api_key:
+                  type: string
+                distinct_id_header:
+                  type: string
+                capture_messages:
+                  type: boolean
+              additionalProperties: false
+              required:
+                - type
+                - url
+                - api_key
     additionalProperties: false
   mode:
     type: string