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.

model/option.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/QuantumNous/new-api/common"
 	"github.com/QuantumNous/new-api/setting"
+	"github.com/QuantumNous/new-api/setting/billing_setting"
 	"github.com/QuantumNous/new-api/setting/config"
 	"github.com/QuantumNous/new-api/setting/operation_setting"
 	"github.com/QuantumNous/new-api/setting/performance_setting"
@@ -121,6 +122,8 @@ func InitOptionMap() {
 	common.OptionMap["UserUsableGroups"] = setting.UserUsableGroups2JSONString()
 	common.OptionMap["CompletionRatio"] = ratio_setting.CompletionRatio2JSONString()
 	common.OptionMap["ImageRatio"] = ratio_setting.ImageRatio2JSONString()
+	common.OptionMap["ModelBillingMode"] = billing_setting.BillingMode2JSONString()
+	common.OptionMap["ModelBillingExpr"] = billing_setting.BillingExpr2JSONString()
 	common.OptionMap["AudioRatio"] = ratio_setting.AudioRatio2JSONString()
 	common.OptionMap["AudioCompletionRatio"] = ratio_setting.AudioCompletionRatio2JSONString()
 	common.OptionMap["TopUpLink"] = common.TopUpLink
@@ -436,6 +439,10 @@ func updateOptionMap(key string, value string) (err error) {
 		err = ratio_setting.UpdateAudioRatioByJSONString(value)
 	case "AudioCompletionRatio":
 		err = ratio_setting.UpdateAudioCompletionRatioByJSONString(value)
+	case "ModelBillingMode":
+		err = billing_setting.UpdateBillingModeByJSONString(value)
+	case "ModelBillingExpr":
+		err = billing_setting.UpdateBillingExprByJSONString(value)
 	case "TopUpLink":
 		common.TopUpLink = value
 	//case "ChatLink":