Add ElevenLabs as a Provider for Text to Speech

[iamdharmesh]

Description of the Change

PR adds ElevenLabs as a provider for the Text to Speech feature.

Closes #993

How to test the Change

1. If you don't have an ElevenLabs account, you'll need to sign up for one (it is free)
2. In your account, create an API key and ensure you have the Text to Speech endpoint enabled and Read access to the Models information (which we use to validate the connection) and Voices
3. Turn on Text to Speech, select ElevenLabs and enter your API key
4. Save settings and ensure no error happens
5. If desired, change the model and voices as per your need
6. Publish post and ensure audio generated properly.
7. Change content of post, enable audio generation from sidebar panel and save post, ensure audio generated properly.

Changelog Entry

Added - ElevenLabs as a Provider for Text to Speech

Credits

Props @dkotter @jeffpaul @iamdharmesh

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added Critical Flows, Test Cases, and/or End-to-End Tests to cover my change.
• All new and existing tests pass.