EventProcessorClient should use DI for blob client (Event Hubs)

[oising]

As per #2870 comments by @eerhardt

We should just be able to call a different overload on the AzureClientFactoryBuilder, which gives us the IServiceProvider. See https://github.com/Azure/azure-sdk-for-net/blob/87d426597b261b88f8a6c6c21fab6031f857fb70/sdk/extensions/Microsoft.Extensions.Azure/src/AzureClientFactoryBuilder.cs#L164-L172. (Note it would take some refactoring of the base AzureComponent<TSettings, TClient, TClientOptions> class, as right now the abstract AddClient method takes a TBuilder, when it could just be passed an AzureClientFactoryBuilder.

I think this needs some configuration-by-convention love in the sense that we should probably only permit a keyed blob client so it can be configured in settings.

I see it a different way. It should allow for a keyed blob client, but shouldn't require it to be keyed. If no "blob service key" was configured, it should just get the BlobServiceClient from DI. If a "blob service key" was specified, then use it to get the BlobServiceClient from DI.

That way the simple case "just works". You just need to:

var builder = WebApplication.CreateBuilder(args);

builder.AddAzureBlobClient("blobConnectionName");
builder.AddAzureEventProcessorClient("eventhubns");
and the EventProcessorClient grabs the BlobClient. There's no need to force someone to use keys when there is only one.