AzurePipelinesCredential does async over sync (over async) causing problems in AMQP tests

AMQP has a specific threading model and gets token on a thread that can't be blocked. It uses async getToken API.
In the case of `AzurePipelinesCredential` this results in something like this

```log
06 Mar 2025 02:34:28,193 [reactor-executor-1] ERROR com.azure.core.http.netty.NettyAsyncHttpClient - block()/blockFirst()/blockLast() are blocking, which is not supported in thread reactor-executor-1
java.lang.IllegalStateException: block()/blockFirst()/blockLast() are blocking, which is not supported in thread reactor-executor-1
at reactor.core@3.4.41/reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:83)
at reactor.core@3.4.41/reactor.core.publisher.Mono.block(Mono.java:1742)
at com.azure.http.netty@1.15.10/com.azure.core.http.netty.NettyAsyncHttpClient.sendSync(NettyAsyncHttpClient.java:195)
at com.azure.core@1.55.2/com.azure.core.http.HttpPipelineNextSyncPolicy.processSync(HttpPipelineNextSyncPolicy.java:51)
at com.azure.core@1.55.2/com.azure.core.http.policy.HttpLoggingPolicy.processSync(HttpLoggingPolicy.java:175)
at com.azure.core@1.55.2/com.azure.core.http.HttpPipelineNextSyncPolicy.processSync(HttpPipelineNextSyncPolicy.java:53)
at com.azure.core@1.55.2/com.azure.core.implementation.http.policy.InstrumentationPolicy.processSync(InstrumentationPolicy.java:101)
at com.azure.core@1.55.2/com.azure.core.http.HttpPipelineNextSyncPolicy.processSync(HttpPipelineNextSyncPolicy.java:53)
at com.azure.core@1.55.2/com.azure.core.http.policy.RetryPolicy.attemptSync(RetryPolicy.java:217)
at com.azure.core@1.55.2/com.azure.core.http.policy.RetryPolicy.processSync(RetryPolicy.java:161)
at com.azure.core@1.55.2/com.azure.core.http.HttpPipelineNextSyncPolicy.processSync(HttpPipelineNextSyncPolicy.java:53)
at com.azure.core@1.55.2/com.azure.core.http.policy.AddHeadersPolicy.processSync(AddHeadersPolicy.java:66)
at com.azure.core@1.55.2/com.azure.core.http.HttpPipelineNextSyncPolicy.processSync(HttpPipelineNextSyncPolicy.java:53)
at com.azure.core@1.55.2/com.azure.core.http.policy.HttpPipelineSyncPolicy.processSync(HttpPipelineSyncPolicy.java:51)
at com.azure.core@1.55.2/com.azure.core.http.policy.UserAgentPolicy.processSync(UserAgentPolicy.java:174)
at com.azure.core@1.55.2/com.azure.core.http.HttpPipelineNextSyncPolicy.processSync(HttpPipelineNextSyncPolicy.java:53)
at com.azure.core@1.55.2/com.azure.core.http.HttpPipeline.sendSync(HttpPipeline.java:138)
at com.azure.identity.AzurePipelinesCredential.lambda$new$0(AzurePipelinesCredential.java:80)
at com.azure.identity.implementation.IdentityClientBase.getConfidentialClient(IdentityClientBase.java:232)
at com.azure.identity.implementation.IdentityClient.lambda$getConfidentialClientApplication$5(IdentityClient.java:152)
at reactor.core@3.4.41/reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:45)
at reactor.core@3.4.41/reactor.core.publisher.MonoCacheTime.subscribeOrReturn(MonoCacheTime.java:143)
at reactor.core@3.4.41/reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:57)
at reactor.core@3.4.41/reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:157)
at reactor.core@3.4.41/reactor.core.publisher.Operators$MonoSubscriber.complete(Operators.java:1839)
at reactor.core@3.4.41/reactor.core.publisher.MonoCacheInvalidateIf$CoordinatorSubscriber.onNext(MonoCacheInvalidateIf.java:319)
at reactor.core@3.4.41/reactor.core.publisher.FluxHandle$HandleSubscriber.onNext(FluxHandle.java:126)
at reactor.core@3.4.41/reactor.core.publisher.SerializedSubscriber.onNext(SerializedSubscriber.java:99)
at reactor.core@3.4.41/reactor.core.publisher.FluxRetryWhen$RetryWhenMainSubscriber.onNext(FluxRetryWhen.java:174)
at reactor.core@3.4.41/reactor.core.publisher.Operators$MonoSubscriber.complete(Operators.java:1839)
at reactor.core@3.4.41/reactor.core.publisher.MonoFlatMap$FlatMapInner.onNext(MonoFlatMap.java:249)
at reactor.core@3.4.41/reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.complete(MonoIgnoreThen.java:292)
at reactor.core@3.4.41/reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.onNext(MonoIgnoreThen.java:187)
at reactor.core@3.4.41/reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.subscribeNext(MonoIgnoreThen.java:236)
at reactor.core@3.4.41/reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.onComplete(MonoIgnoreThen.java:203)
at reactor.core@3.4.41/reactor.core.publisher.FluxPeek$PeekSubscriber.onComplete(FluxPeek.java:260)
at reactor.core@3.4.41/reactor.core.publisher.SerializedSubscriber.onComplete(SerializedSubscriber.java:146)
at reactor.core@3.4.41/reactor.core.publisher.FluxTimeout$TimeoutMainSubscriber.onComplete(FluxTimeout.java:234)
at reactor.core@3.4.41/reactor.core.publisher.FluxSwitchIfEmpty$SwitchIfEmptySubscriber.onComplete(FluxSwitchIfEmpty.java:85)
at reactor.core@3.4.41/reactor.core.publisher.MonoNext$NextSubscriber.onComplete(MonoNext.java:102)
at reactor.core@3.4.41/reactor.core.publisher.MonoNext$NextSubscriber.onNext(MonoNext.java:83)
at reactor.core@3.4.41/reactor.core.publisher.FluxFilter$FilterSubscriber.onNext(FluxFilter.java:113)
at reactor.core@3.4.41/reactor.core.publisher.EmitterProcessor.drain(EmitterProcessor.java:537)
at reactor.core@3.4.41/reactor.core.publisher.EmitterProcessor.tryEmitNext(EmitterProcessor.java:343)
at reactor.core@3.4.41/reactor.core.publisher.SinkManySerialized.tryEmitNext(SinkManySerialized.java:100)
at reactor.core@3.4.41/reactor.core.publisher.InternalManySink.emitNext(InternalManySink.java:27)
at com.azure.core.amqp.implementation.RequestResponseChannel.updateEndpointState(RequestResponseChannel.java:496)
```

because of this code

https://github.com/Azure/azure-sdk-for-java/blob/fe75c7d72a3561ac83cb1efedeeea2dcb68e6538/sdk/identity/azure-identity/src/main/java/com/azure/identity/AzurePipelinesCredential.java#L80

that does sync over async (and back to async under because of Netty).

While this affects tests only, it'd be nice to avoid async over sync and minimize issues like this.