Add NamingSource.FromHandlerType for conventional routing

[jeremydmiller]

Summary

Closes #2359

Adds NamingSource.FromHandlerType — an alternative naming mode for conventional routing that names listener queues/topics after the handler type instead of the message type. This is useful for modular monolith scenarios where multiple handlers exist for the same message type and each needs its own dedicated queue to receive messages independently.

Changes

• New NamingSource enum with FromMessageType (default, existing behavior) and FromHandlerType
• MessageRoutingConvention base class — added UseNaming(NamingSource) method and modified DiscoverListeners() to create per-handler-type listener endpoints when FromHandlerType is selected, with StickyHandlers to ensure only the matching handler processes messages from each queue
• New UseConventionalRouting(NamingSource, ...) overloads on all transports with conventional routing:
  • RabbitMQ
  • Azure Service Bus (queue and topic/subscription variants)
  • AWS SQS
  • GCP Pub/Sub
• Tests for each transport verifying handler-type-based naming
• Documentation updated for all 4 transports explaining the new option and when to use it

Usage

opts.UseRabbitMq()
    .UseConventionalRouting(NamingSource.FromHandlerType);

Test plan

• 3 new RabbitMQ handler type naming tests pass
• All 26 RabbitMQ conventional routing tests pass (no regressions)
• All 1196 CoreTests pass
• All 4 transport test projects build successfully (Azure SB, AWS SQS, GCP Pub/Sub, RabbitMQ)

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