Completion API: add new params (#870)

* Completion API: add 'store' param

This param allows you to opt a completion request in to being stored,
for use in distillations and evals.

* Add cached and audio tokens to usage structs

These have been added to the completions API recently:
https://platform.openai.com/docs/api-reference/chat/object#chat/object-usage

common.go

@@ -7,10 +7,18 @@ type Usage struct {
 	PromptTokens            int                      `json:"prompt_tokens"`
 	CompletionTokens        int                      `json:"completion_tokens"`
 	TotalTokens             int                      `json:"total_tokens"`
+	PromptTokensDetails     *PromptTokensDetails     `json:"prompt_tokens_details"`
 	CompletionTokensDetails *CompletionTokensDetails `json:"completion_tokens_details"`
 }
 
 // CompletionTokensDetails Breakdown of tokens used in a completion.
 type CompletionTokensDetails struct {
+	AudioTokens     int `json:"audio_tokens"`
 	ReasoningTokens int `json:"reasoning_tokens"`
 }
+
+// PromptTokensDetails Breakdown of tokens used in the prompt.
+type PromptTokensDetails struct {
+	AudioTokens  int `json:"audio_tokens"`
+	CachedTokens int `json:"cached_tokens"`
+}