feat(sdk-core): add Async HTTP-client warm-up to SdkWarmUp.prime() for CRaC priming

[joviegas]

Motivation and Context

Async counterpart to #7080 (sync warm-up). SdkWarmUp.prime() warmed only sync HTTP clients.
Similar implementation was needed for async clients (Netty, CRT)

Modifications

Adds AsyncHttpClientWarmer, the async analogue of SyncHttpClientWarmer from #7080:
discovers each SdkAsyncHttpService, sends a best-effort GET to the STS endpoint, drains
the response. Reuses the same infra (HttpClientWarmer, WarmUpDiscovery,
RegionEndpointResolver) and is wired into ClasspathHttpWarmupInvoker, so prime() now
warms both sync and async.

Async-specific (vs #7080):

• Bounded 5s block on the async CompletableFuture — must complete before checkpoint.
• Drains the reactive body via SimpleSubscriber (overrides no-op onComplete/onError).
• Settles on the execute future too, covering failures with no body signal (no hang).
• Always closes the client (no leaked FDs → CRaC-safe).
• SpotBugs ASYNC_BLOCKING_CALL suppression for the intentional bounded get().

Test

• Junit test added .

License

• I confirm that this pull request can be released under the Apache 2 license

[github-actions]

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