feat: add plugin metadata endpoint and reorganize test files

[eveld]

Summary

This PR adds plugin metadata endpoint functionality to HCLConfig and reorganizes test files to follow Go conventions.

Changes

Plugin Metadata Support

• New Metadata struct with plugin information:
  • Name, version, description, author, homepage, license
  • Capabilities (slice of strings)
  • API version and supported OS/architecture
• Added Metadata() method to Plugin interface
• Updated PluginBase with metadata storage and accessor methods

Plugin Implementations Updated

• TestPlugin and EmbeddedTestPlugin in test_plugin.go
• PersonPlugin in plugins/example/main.go
• SimpleTestPlugin in parser tests
• All implementations include proper metadata with "v" prefix for semantic versions

Test File Reorganization

• Moved plugin discovery utilities from testutils_discovery_test.go to testutils_test.go
• Moved parser plugin tests from parser_plugin_test.go to parser_test.go
• Added comprehensive metadata tests in plugins/plugin_test.go
• Removed separate test utility files following Go conventions

Key Features

• Metadata can be retrieved without plugin initialization - perfect for registry systems
• JSON serializable - ready for API responses
• Backward compatible - all existing tests pass
• Semantic versioning with "v" prefix for consistency

Test Results

All tests pass including:

• Metadata JSON serialization/deserialization
• Plugin interface compliance
• Pre-initialization metadata access
• Plugin discovery functionality
• Parser plugin registration

Usage Example

plugin := &TestPlugin{}
metadata := plugin.Metadata()
// Returns: name="test", version="v1.0.0", api="v1", etc.

This provides the foundation for plugin registry systems to query plugin metadata programmatically without full initialization.