[Roadmap] Support ElevenLabs Custom API Endpoint

[Falcon7382]

Why
We use proxies for all our AI endpoints and the lack of support for ElevenLabs Custom Endpoint within Big-AGI has prevented us from being able to utilise the significant benefits that come with using it.

Description
A advanced settings for ElevenLabs that allows users to enter a Custom Endpoint, if not provided then prepopulate with default endpoint.