feat: implement tiered billing expression evaluation and related functionality

- Added support for tiered billing expressions in the billing system.
- Introduced new types and functions for handling billing expressions, including caching and execution.
- Updated existing billing logic to accommodate tiered billing scenarios.
- Enhanced request handling to support incoming billing expression requests.
- Added tests for tiered billing functionality to ensure correctness.

relay/chat_completions_via_responses.go

@@ -2,6 +2,7 @@ package relay
 
 import (
 	"bytes"
+	"io"
 	"net/http"
 	"strings"
 
@@ -124,8 +125,10 @@ func chatCompletionsViaResponses(c *gin.Context, info *relaycommon.RelayInfo, ad
 		return nil, types.NewError(err, types.ErrorCodeConvertRequestFailed, types.ErrOptionWithSkipRetry())
 	}
 
+	var requestBody io.Reader = bytes.NewBuffer(jsonData)
+
 	var httpResp *http.Response
-	resp, err := adaptor.DoRequest(c, info, bytes.NewBuffer(jsonData))
+	resp, err := adaptor.DoRequest(c, info, requestBody)
 	if err != nil {
 		return nil, types.NewOpenAIError(err, types.ErrorCodeDoRequestFailed, http.StatusInternalServerError)
 	}