Adding the changelog mentioning the HeyGen changes.

changelog/3210.added.md

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+- Added support for the HeyGen LiveAvatar API
+  (see https://www.liveavatar.com/).

changelog/3210.changed.md

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+- Updated `HeyGenVideoService` and `HeyGenTransport` to support both HeyGen APIs (Interactive Avatar and Live Avatar).
+  Using them is as simple as specifying the `service_type` when creating the `HeyGenVideoService` and the `HeyGenTransport`:
+  ```python
+  heyGen = HeyGenVideoService(
+      api_key=os.getenv("HEYGEN_LIVE_AVATAR_API_KEY"),
+      service_type=ServiceType.LIVE_AVATAR,
+      session=session,
+  )
+  ```
+