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/round.go

@@ -0,0 +1,10 @@
+package billingexpr
+
+import "math"
+
+// QuotaRound converts a float64 quota value to int using half-away-from-zero
+// rounding. Every tiered billing path (pre-consume, settlement, breakdown
+// validation, log fields) MUST use this function to avoid +-1 discrepancies.
+func QuotaRound(f float64) int {
+	return int(math.Round(f))
+}