feat: add experimental_bearer_token option to model provider definition (#5467)

While we do not want to encourage users to hardcode secrets in their
`config.toml` file, it should be possible to pass an API key
programmatically. For example, when using `codex app-server`, it is
possible to pass a "bag of configuration" as part of the
`NewConversationParams`:

682d05512f/codex-rs/app-server-protocol/src/protocol.rs (L248-L251)

When using `codex app-server`, it's not practical to change env vars of
the `codex app-server` process on the fly (which is how we usually read
API key values), so this helps with that.

codex-rs/core/src/config.rs

@@ -2801,6 +2801,7 @@ model_verbosity = "high"
             env_key: Some("OPENAI_API_KEY".to_string()),
             wire_api: crate::WireApi::Chat,
             env_key_instructions: None,
+            experimental_bearer_token: None,
             query_params: None,
             http_headers: None,
             env_http_headers: None,