feat: plugin architecture for custom test attributes and third-party integration

[thomhurst]

Description

The 58 interfaces in TUnit.Core/Interfaces/ provide extension points, but there's no extensible attribute system for third-party tool integration. Users can't easily create custom test metadata attributes that integrate into the core pipeline.

Why

• Tool vendors can't add native test attributes
• No standard way for team-specific test metadata
• Every custom attribute requires modifying TUnit core

Proposed Approach

• Create ICustomTestAttribute interface with standardized lifecycle
• Implement plugin discovery via MEF/reflection with caching
• Add --plugin-path command-line option
• Create validation rules for custom attributes (analyzer support)
• Document plugin contract guarantees and version compatibility