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
 }