[Feature] Synchronized version of AwsCrt4aSigningAdapter

### Describe the feature

The feature request is for an implementation of the `AwsCrt4aSigningAdapter` that either uses synchronized signing method or employs non-daemon threads during signing operations.

https://github.com/aws/aws-sdk-java-v2/blob/80e1821456aea846fca72ee6d0ffdb8b82a3ea48/core/auth-crt/src/main/java/software/amazon/awssdk/authcrt/signer/internal/AwsCrt4aSigningAdapter.java#L48

### Use Case

The current SigV4a signer is implemented asynchronously using `CompletableFuture`, which defaults to using daemon thread. However, in specific scenarios such as when Spark's `ShutdownHookManager` is active, these daemon threads can be prematurely interrupted, leading to potential issues.

### Proposed Solution

An implementation of the `AwsCrtV4aSigner` that either uses synchronized signing method or employs non-daemon threads during signing operations.

Pseudo code
```
public class AwsSigner {
private static final ThreadFactory nonDaemonThreadFactory = new ThreadFactory() {
private final AtomicInteger threadCount = new AtomicInteger(0);

@Override
public Thread newThread(Runnable r) {
Thread t = new Thread(r);
t.setDaemon(false); // Set the thread as non-daemon
t.setName("AwsSignerThread-" + threadCount.getAndIncrement());
return t;
}
};

/**
* Signs an http request according to the supplied signing configuration
* @param request http request to sign
* @param config signing configuration
* @return future which will contain the signed request
*/
static public CompletableFuture signRequest(HttpRequest request, AwsSigningConfig config) {
CompletableFuture future = new CompletableFuture();
CompletableFuture result = AwsSigner.sign(request, config);

result.whenCompleteAsync((res, throwable) -> {
if (throwable != null) {
future.completeExceptionally(throwable);
} else {
future.complete(res.getSignedRequest());
}
}, Executors.newSingleThreadExecutor(nonDaemonThreadFactory));
return future;
}
}
```

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

Start with core/auth-crt/src/main/java/software/amazon/awssdk/authcrt/signer/internal/AwsCrt4aSigningAdapter.java at the referenced signing path, then inspect how AwsCrtV4aSigner completes its CompletableFuture. Determine whether synchronized signing or non-daemon execution fits the existing design, and define completion around signing surviving the described shutdown-hook scenario without breaking request results.

Written by the indexing model from the issue text.