Add initial extstore types

[cconstable]

What was changed

Adds the @Experimental External Storage API surface under io.temporal.payload.storage. Type
definitions only.

Types:

• StorageDriver: driver contract (async store/retrieve, getName/getType).
• StorageDriverClaim: claim-check token for an externally stored payload.
• StorageDriverSelector: chooses a driver per payload, or keeps it inline.
• StorageDriverStoreContext / StorageDriverRetrieveContext: driver-call contexts.
• StorageDriverTargetInfo plus StorageDriverWorkflowInfo / StorageDriverActivityInfo: best-effort,
  store-side target identity.
• ExternalStorage (plus Builder): config for drivers, optional selector, and size threshold.

Notes:

• No WorkflowClientOptions setter yet.
• Builder validation throws IllegalStateException. Stable diagnostic codes are a follow-up.

Why?

• First part of part of Large Payloads -> External Storage work

Checklist

• Added tests.

[cconstable]

Right now, these all throw state exceptions (which are tested). In a subsequent PR, I would like to replace these with stable diagnostic codes.

[jmaeagle99]

For these in particular, do we need diagnostic codes? The messages are fairly clear as to what the issue is and prevent the use of external storage at startup rather than having to report during execution runtime.

[cconstable]

I am a proponent of best-effort, stable diagnostic codes if the mechanism for adding them very easy. I have a little write-up about this that I wanted to share but the tl;dr is I think we should make a best effort to tag anything that's actionable if for nothing else it supports automated tooling (e.g. AI) and feels like a healthy industry trend (hand wavy gesture). I would not have felt this way 4 years ago.

[jmaeagle99]

Left some comments but overall looks good. Please hold off until a Java owner reviews and approves.