Improved cost transparency with usage-based billing

Introduces the Temporal Cloud Billing API for namespace- and tag-level cost attribution with hourly granularity, and the Billable Action Count metric for granular usage information (1-minute granularity). Details how report generation requests are mapped to Temporal Workflows, using Workflow Queries for status, Signals for queuing, and Child Workflows for processing. Implements account-level queuing and global rate-limiting using these Temporal primitives to manage concurrency and fairness with backend data sources that have low rate limits.