feat: external storage foundation types

[jmaeagle99]

What was changed

Add foundational external storage types to the Ruby SDK, marked @!visibility private and unwired
from the rest of the SDK.

• Temporalio::Converters::StorageDriver — base class users subclass to store and retrieve payloads
  in an external system, alongside StorageDriverClaim, the workflow/activity target info types, and
  the store, select and retrieve contexts.
• Temporalio::Converters::ExternalStorage — configuration, validated at construction: at least one
  driver, unique non-empty driver names, a selector required above one driver, and a non-negative
  size threshold.
• Temporalio::Internal::ExternalStorageReferences — the on-the-wire reference format.
• Temporalio::Error::ExternalStorageNotConfiguredError — TMPRL1105.
• RBS and RBI signatures for all of the above, plus unit tests and a shared in-memory fake driver.

Nothing calls any of this yet. There is no runner, no data converter integration, and no worker or
client wiring.

Why?

Allow parallel development of the external storage system in the Ruby SDK. Keeping the types private
and unwired prevents unintended use.

Notes for Reviewers

• StorageDriver#store and #retrieve are synchronous and may block on I/O, following
  PayloadCodec. A driver that wants to work its batch concurrently should fan out with threads,
  not Fiber.schedule — the latter raises No scheduler is available! when the worker is configured
  with a payload codec thread pool rather than a fiber scheduler, so a fiber-based driver works under
  one supported worker configuration and crashes under the other.
• StorageDriverClaim defines ==, eql? and hash over its claim data. Java and Python both give
  claims content equality, and identity comparison would make two claims describing the same stored
  payload unequal.
• Concurrency controls for driver operations are deliberately out of scope here.

Checklist

1. How was this tested: New unit tests

2. Any docs updates needed? No. The types are @!visibility private and omitted from CHANGELOG.md
   until external storage is usable.