feat: NRT support

[pchalamet]

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[Copilot]

Pull Request Overview

This PR introduces Nullable Reference Types (NRT) support to the FBus library by enabling nullable annotations and updating the codebase to be compatible with nullable reference types.

• Added NRT support through Directory.Build.props configuration
• Refactored message interfaces by introducing a base IMessage interface
• Updated type annotations throughout the codebase to properly handle null values

Reviewed Changes

Copilot reviewed 22 out of 22 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
Directory.Build.props	Enables nullable reference types for the entire project
FBus/FBus.fs	Refactors message interfaces and adds nullable annotations to core types
FBus/Control.fs	Updates message handling logic with proper null handling patterns
FBus/Serializers.fs	Removes explicit type annotation from Serialize method parameter
FBus.RabbitMQ7/Transport.fs	Adds null checking for RabbitMQ headers
FBus.Json/Serializer.fs	Updates serialization with null checking
FBus.GenericHost/Container.fs	Updates container interface with nullable return types
FBus.Extensions.Tests/FBusExtensions.cs	Updates C# interface implementations with nullable annotations
Multiple test files	Updates message types to implement new interface hierarchy
Multiple sample files	Updates consumers and message types for new interface structure

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