agent: Add ability to change the API base URL for OpenAI via the UI (#32979)

The `api_url` setting is one that most providers already support and can
be changed via the `settings.json`. We're adding the ability to change
it via the UI for OpenAI specifically so it can be more easily connected
to v0.

Release Notes:

- agent: Added ability to change the API base URL for OpenAI via the UI

---------

Co-authored-by: Bennet Bo Fenner <53836821+bennetbo@users.noreply.github.com>

Cargo.lock

@@ -9008,6 +9008,7 @@ dependencies = [
  "tiktoken-rs",
  "tokio",
  "ui",
+ "ui_input",
  "util",
  "workspace-hack",
  "zed_llm_client",