feat: add external storage concurrency limiter APIs

[jmaeagle99]

What was changed

Adds the concurrency contract for external storage drivers:

• StorageDriverLimiter, with permit(item) { ... } holding a permit for the block.
• ExternalStorageConcurrency, carrying the two budgets: max_driver_operations (64, shared
  across the configuration) and max_operations_per_message (8, per activation/task/request).
• The limiter is now on StorageDriverStoreContext and StorageDriverRetrieveContext, and
  ExternalStorage takes a concurrency:.

Why?

A driver is handed a whole batch and decides for itself how many requests to make of it, so the SDK can only bound that fan out cooperatively by handing the driver something to take a permit from. The spec puts the limiter on the store and retrieve contexts and the budgets on the storage configuration.

Ruby's block form is the counterpart of Python's async with limiter.permit(item): — both hold the permit for a lexical scope and release it on exit, including when the body raises. Field names and defaults match Python's ExternalStorageConcurrency exactly. No cancellation parameter is needed because the Ruby contexts already carry a Cancellation.

Checklist

1. How was this tested: New unit tests cover the defaults (64 / 8), rejection of non-positive and non-Integer budgets, and that permit runs the block and returns its value.

2. Any docs updates needed? Not yet. Everything here is @!visibility private and experimental.

[chris-olszewski]

Just small questions around some doc comment language