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/helper/billing_expr_request.go

@@ -0,0 +1,54 @@
+package helper
+
+import (
+	"strings"
+
+	"github.com/QuantumNous/new-api/common"
+	"github.com/QuantumNous/new-api/pkg/billingexpr"
+	relaycommon "github.com/QuantumNous/new-api/relay/common"
+	"github.com/gin-gonic/gin"
+)
+
+func ResolveIncomingBillingExprRequestInput(c *gin.Context, info *relaycommon.RelayInfo) (billingexpr.RequestInput, error) {
+	input := billingexpr.RequestInput{}
+	if info != nil {
+		input.Headers = cloneStringMap(info.RequestHeaders)
+	}
+
+	bodyBytes, err := readIncomingBillingExprBody(c)
+	if err != nil {
+		return billingexpr.RequestInput{}, err
+	}
+	input.Body = bodyBytes
+	return input, nil
+}
+
+func readIncomingBillingExprBody(c *gin.Context) ([]byte, error) {
+	if c == nil || c.Request == nil || !isJSONContentType(c.Request.Header.Get("Content-Type")) {
+		return nil, nil
+	}
+	storage, err := common.GetBodyStorage(c)
+	if err != nil {
+		return nil, err
+	}
+	return storage.Bytes()
+}
+
+func isJSONContentType(contentType string) bool {
+	contentType = strings.ToLower(strings.TrimSpace(contentType))
+	return strings.HasPrefix(contentType, "application/json")
+}
+
+func cloneStringMap(src map[string]string) map[string]string {
+	if len(src) == 0 {
+		return map[string]string{}
+	}
+	dst := make(map[string]string, len(src))
+	for key, value := range src {
+		if strings.TrimSpace(key) == "" {
+			continue
+		}
+		dst[key] = value
+	}
+	return dst
+}