Allow OpenAI API URL to be configured via assistant.openai_api_url (#7552)

Partially fixes #4321, since Azure OpenAI API can be converted to OpenAI
API.

Release Notes:

- Added `assistant.openai_api_url` setting to allow OpenAI API URL to be
configured.

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/assistant/src/assistant.rs

@@ -68,6 +68,7 @@ struct SavedConversation {
     messages: Vec<SavedMessage>,
     message_metadata: HashMap<MessageId, MessageMetadata>,
     summary: String,
+    api_url: Option<String>,
     model: OpenAiModel,
 }