Brave to allow users to use their own AI model

[Encounter5729]

You will need to connect to an endpoint, not sure if it will.work with local models.

https://github.com/brave/brave-core/pull/23916/files/c2d157b00ae6ced760ba686ac54aa563bd8bbd9d#diff-acea01894150288ed5a7f3b8bdca1c564e1290a8aeb665b5be520033ed8c57c0