[FEATURE REQ] Allow configuration of Reactive Scheduler on SDK or Client level

[bca111]

Is your feature request related to a problem? Please describe.

My application has a dedicated thread pool for IO operations that I want to use for azure-sdk async clients (namely eventhubs and blob storage ones) as a Scheduler instead of Schedulers.boundedElastic() but I couldn't find any way to set it across the sdk, or client specific. EventHubClientBuilder has a EventHubClientBuilder scheduler(Scheduler scheduler) method but it's not public. BlobServiceClientBuilder doesn't have such method.

Describe the solution you'd like
Some way to configure Scheduler for all SDK or client basis. Such as

EventHubClientBuilder()
           .connectionString(...)
           .scheduler(customScheduler) // method exist but package private
           .buildAsyncProducerClient()

BlobServiceClientBuilder()
           .scheduler(customScheduler) // method doesn't exist
           .buildAsyncClient()

Describe alternatives you've considered
I'm currently using Mono<T> publishOn(Scheduler scheduler) method but it's kind of error prone. Also I'm not sure that's enough to avoid creation of the Schedulers.boundedElastic() pool for consumer clients. Here's how my code looks now:

eventHubProducerClient.send(data, sendOptions)
               .publishOn(customScheduler)
               .subscribe(...)

Additional context
N/A

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