🌿 ✨ [Scheduled] Update API Spec (#58)

* feat(openapi): add Langfuse credential support and new voice IDs

This commit adds support for Langfuse credentials, including the required fields (`provider`, `publicKey`, `apiKey`, `apiUrl`) and the unique identifiers (`id` and `orgId`). It also introduces new voice IDs: `ash`, `ballad`, `coral`, `sage`, and `verse`, which are exclusive to the `gpt-4o-realtime-preview-2024-10-01` model.

* feat(voice): add support for voice provider fallbacks and increase timeouts

* fix(fern/apis/api/openapi.json): Updated openapi.json with new transfer mode and SIP header support

This commit introduces a new transfer mode, "blind-transfer-add-summary-to-sip-header", which adds a SIP header with a call summary during a blind transfer. It also adds a new "sipHeaders" object to allow custom headers during a transfer call. Additionally, a new Azure region, "uaenorth", has been added to the list of supported regions.

* fix(fern/apis/api/openapi.json): update transfer mode descriptions and remove unused sipHeaders property

* feat(openapi): add AssemblyAI transcriber and update voice providers

---------

Co-authored-by: fern-api[bot] <115122769+fern-api[bot]@users.noreply.github.com>

Author: fern-api[bot]