support base_url path for model providers (#608)

* adding support for base_url

* updated docs

* fixed tests for config generator

* making fixes based on PR comments

---------

Co-authored-by: Salman Paracha <salmanparacha@MacBook-Pro-288.local>

crates/llm_gateway/src/stream_context.rs

@@ -121,6 +121,7 @@ impl StreamContext {
                     .as_ref()
                     .unwrap_or(&"".to_string()),
                 self.streaming_response,
+                self.llm_provider().base_url_path_prefix.as_deref(),
             );
             if target_endpoint != request_path {
                 self.set_http_request_header(":path", Some(&target_endpoint));