[Cosmos] Client-side throughput control

[tvaron3]

Summary

Add client-side throughput control to the Cosmos Rust SDK, complementing the server-side features (priority-based execution and throughput buckets) shipped in #4078.

Scope

Re-introduce the ClientSide variant to ThroughputControlGroupOptions (removed in #4078 for GA, enum is #[non_exhaustive]):

• ThroughputTarget enum — Absolute(u32) for fixed RU/s limits, Threshold(f64) for percentage of provisioned throughput
• Client-side rate limiter — SDK-enforced request rate limiting before sending to the service
• Optional priority level — combine with client-side throttling for local prioritization

References

• Java SDK: ThroughputControlGroupConfigBuilder
• Parent issue: [Cosmos] ThroughputControl API #3904
• Server-side implementation: [Cosmos] ThroughputControl API #4078