feat!(extstore): add new concurrency controls for external storage.

[cconstable]

What was changed

ExternalStorage.Builder.setConcurrency takes two limits:

• maxDriverOperations caps requests in flight for a single client or worker. Each client and each worker gets its own limit.
• maxOperationsPerMessage caps requests for a single message (workflow task activation, etc). Each message gets its own budget.

Both limits are cooperative. Drivers receive a limiter on the store and retrieve contexts and must wrap each request in permit. Requests made outside a permit are not counted, and a warning is logged when a driver completes without taking one.

context.getLimiter().permit(payload, () -> {
    // store/retrieve logic
});

Removed ExternalStorage.Builder.setMaxConcurrentPayloadVisits. The payload visitor now walks a message's payload sites maxOperationsPerMessage at a time. The bundled S3 driver takes permits.

Why?

The previous control bounded payload visits rather than driver calls, so setting it to 3 did not limit external storage requests to 3. It also only applied to workflow tasks, leaving client, activity, and Nexus paths unbounded.

maxDriverOperations is scoped to a client or worker so that ExternalStorage stays configuration. Holding the counter there meant two clients built from one config silently shared a limit.

Checklist

Added tests