Simplify RabbitMQ setup and documentation

[atakanatali]

Motivation

• Make UseRabbitMq() the single-call integration for RabbitMQ so samples and consumers no longer need manual option binding or singleton registrations.
• Prefer binding broker settings from Notify:Broker:RabbitMq while remaining compatible with a legacy RabbitMq root section.
• Resolve queue prefix automatically from NotifyOptions.QueuePrefix or IHostEnvironment.ApplicationName to simplify sample code.
• Ensure the standard queues (email/sms/push) are declared at startup to avoid manual topology setup in samples.

Description

• INotifyBuilder.UseRabbitMq now registers RabbitMqOptions binding via a new RabbitMqOptionsConfigurator, applies optional overrides with PostConfigure, and registers a singleton RabbitMqBrokerClient and corresponding IBrokerPublisher, IBrokerConsumer, and IBrokerClient mappings.
• Added ResolveRabbitMqSection and ResolveQueuePrefix logic and the nested RabbitMqOptionsConfigurator to bind from Notify:Broker:RabbitMq with a fallback to RabbitMq, and to pick a queue prefix from NotifyOptions or host name.
• RabbitMqBrokerClient was extended with EnsureStandardQueues() and topology declaration improvements so standard queues are created when the client is instantiated.
• Updated sample apps and README.md to remove manual RabbitMqOptions binding and singleton registrations and document the new Notify:Broker:RabbitMq config path; updated project references (Notify.Core / Notify.Broker.RabbitMQ csproj changes) and removed the empty RabbitMqOptions from Notify.Core.