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.

pkg/billingexpr/settle.go

@@ -0,0 +1,24 @@
+package billingexpr
+
+// ComputeTieredQuota runs the Expr from a frozen BillingSnapshot against
+// actual token counts and returns the settlement result.
+func ComputeTieredQuota(snap *BillingSnapshot, params TokenParams) (TieredResult, error) {
+	return ComputeTieredQuotaWithRequest(snap, params, RequestInput{})
+}
+
+func ComputeTieredQuotaWithRequest(snap *BillingSnapshot, params TokenParams, request RequestInput) (TieredResult, error) {
+	cost, trace, err := RunExprByHashWithRequest(snap.ExprString, snap.ExprHash, params, request)
+	if err != nil {
+		return TieredResult{}, err
+	}
+
+	afterGroup := QuotaRound(cost * snap.GroupRatio)
+	crossed := trace.MatchedTier != snap.EstimatedTier
+
+	return TieredResult{
+		ActualQuotaBeforeGroup: cost,
+		ActualQuotaAfterGroup:  afterGroup,
+		MatchedTier:            trace.MatchedTier,
+		CrossedTier:            crossed,
+	}, nil
+}