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/common/billing.go

@@ -18,4 +18,7 @@ type BillingSettler interface {
 
 	// GetPreConsumedQuota 返回实际预扣的额度值（信任用户可能为 0）。
 	GetPreConsumedQuota() int
+
+	// Reserve 将预扣额度补到目标值；若目标值不高于当前预扣额度则不做任何事。
+	Reserve(targetQuota int) error
 }

relay/common/relay_info.go

@@ -10,6 +10,7 @@ import (
 	"github.com/QuantumNous/new-api/common"
 	"github.com/QuantumNous/new-api/constant"
 	"github.com/QuantumNous/new-api/dto"
+	"github.com/QuantumNous/new-api/pkg/billingexpr"
 	relayconstant "github.com/QuantumNous/new-api/relay/constant"
 	"github.com/QuantumNous/new-api/setting/model_setting"
 	"github.com/QuantumNous/new-api/types"
@@ -152,6 +153,11 @@ type RelayInfo struct {
 
 	PriceData types.PriceData
 
+	// TieredBillingSnapshot is a frozen snapshot of tiered billing rules
+	// captured at pre-consume time. Non-nil only when billing mode is "tiered_expr".
+	TieredBillingSnapshot *billingexpr.BillingSnapshot
+	BillingRequestInput   *billingexpr.RequestInput
+
 	Request dto.Request
 
 	// RequestConversionChain records request format conversions in order, e.g.