[SDK-2] Plugin contract: [FalloutPlugin] attribute + IFalloutPlugin

[ChrisonSimtian]

Implement the plugin contract as proposed in RFC #1.

Scope

• [assembly: FalloutPlugin(typeof(MyPlugin))] attribute.
• IFalloutPlugin interface — single Configure(IServiceCollection) (final shape per RFC).
• Validation: clear errors when attribute points at a type that doesn't implement the interface or has no public parameterless ctor.

Depends on

SDK-1 (#146).