feat: add RabbitPropsCustomizer and KafkaPropsCustomizer for flexible multi-domain configuration

[luisgomez29]

Add support for RabbitPropsCustomizer and KafkaPropsCustomizer interfaces to enable
programmatic customization of domain properties at startup, eliminating the requirement
for a mandatory "app" domain. Document the three configuration approaches with examples
and guidance.

What's New

RabbitPropsCustomizer & KafkaPropsCustomizer

• New feature: Customize async properties programmatically while keeping YAML configuration
• Use case: Load connection credentials from secrets manager at startup, override YAML values
• Constraint: At least one domain must be declared in YAML (if using customizer)
• Benefit: Hybrid approach combines simplicity of YAML with runtime flexibility of code

No Longer Required: "app" Domain

• The "app" domain is now optional when using a custom default domain name
• First domain declared in YAML becomes the default (receives HandlerRegistry.DEFAULT_DOMAIN alias)
• Handlers registered without explicit domain resolve to the first configured domain
• Updated documentation to clarify this behavior