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.

.gitignore

@@ -29,3 +29,6 @@ data/
 .gomodcache/
 .gocache-temp
 .gopath
+
+token_estimator_test.go
+skills-lock.json