fix: Update AWS adapter to handle stream completions and calculate usage metrics

Based on the file summaries provided, here are the important bullet points for the commit message:

- Add functionality to handle stream completion events from AWS in the relay/adaptor/aws/main.go file
- Marshall AWS response to OpenAI format and calculate usage metrics in the same file
- Implement a custom render function for streaming events in the same file
- Improve error handling for JSON unmarshalling and marshalling errors in the same file

relay/adaptor/aws/main.go

@@ -188,6 +188,7 @@ func StreamHandler(c *gin.Context, resp *http.Response) (*relaymodel.ErrorWithSt
 	c.Stream(func(w io.Writer) bool {
 		event, ok := <-stream.Events()
 		if !ok {
+			c.Render(-1, common.CustomEvent{Data: "data: [DONE]"})
 			return false
 		}