Feat: Add assistant usage (#649)

* Feat: Add assistant usage

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Co-authored-by: shadowpigy <shadowpigy@github.com>

run.go

@@ -26,6 +26,7 @@ type Run struct {
 	Tools          []Tool             `json:"tools"`
 	FileIDS        []string           `json:"file_ids"`
 	Metadata       map[string]any     `json:"metadata"`
+	Usage          Usage              `json:"usage,omitempty"`
 
 	httpHeader
 }