Add generic config field framework

pkg/config/doc.go

@@ -0,0 +1,110 @@
+// Package config provides configuration management for ToolHive, including a
+// generic framework for easily adding new configuration fields.
+//
+// # Architecture
+//
+// The package uses a Provider pattern to abstract configuration storage:
+//   - DefaultProvider: Uses XDG config directories (~/.config/toolhive/config.yaml)
+//   - PathProvider: Uses a specific file path (useful for testing)
+//   - KubernetesProvider: No-op implementation for Kubernetes environments
+//
+// # Generic Config Field Framework
+//
+// The framework allows you to define config fields declaratively with minimal
+// boilerplate. Fields are registered once with validation, getters, setters,
+// and unseters.
+//
+// # Adding a New Config Field
+//
+// Step 1: Add your field to the Config struct:
+//
+//	type Config struct {
+//	    // ... existing fields ...
+//	    MyNewField string `yaml:"my_new_field,omitempty"`
+//	}
+//
+// Step 2: Register the field using a helper constructor:
+//
+//	func init() {
+//	    // For simple string fields:
+//	    config.RegisterStringField("my-field",
+//	        func(cfg *Config) *string { return &cfg.MyNewField },
+//	        validateMyField) // Optional validator
+//
+//	    // For boolean fields:
+//	    config.RegisterBoolField("my-bool-field",
+//	        func(cfg *Config) *bool { return &cfg.MyBoolField },
+//	        nil) // nil = no validation
+//
+//	    // For float fields:
+//	    config.RegisterFloatField("my-float-field",
+//	        func(cfg *Config) *float64 { return &cfg.MyFloatField },
+//	        0.0, // zero value
+//	        validateMyFloat)
+//
+//	    // For string slice fields (comma-separated):
+//	    config.RegisterStringSliceField("my-list-field",
+//	        func(cfg *Config) *[]string { return &cfg.MyListField },
+//	        nil)
+//	}
+//
+// Step 3: Use the field through the generic framework:
+//
+//	provider := config.NewDefaultProvider()
+//
+//	// Set a value
+//	err := config.SetConfigField(provider, "my-field", "some-value")
+//
+//	// Get a value
+//	value, isSet, err := config.GetConfigField(provider, "my-field")
+//
+//	// Unset a value
+//	err := config.UnsetConfigField(provider, "my-field")
+//
+// # Advanced: Custom Field Registration
+//
+// For fields with complex logic, use RegisterConfigField directly:
+//
+//	config.RegisterConfigField(config.ConfigFieldSpec{
+//	    Name: "my-complex-field",
+//	    SetValidator: func(_ Provider, value string) error {
+//	        // Custom validation logic
+//	        return nil
+//	    },
+//	    Setter: func(cfg *Config, value string) {
+//	        // Custom setter logic
+//	    },
+//	    Getter: func(cfg *Config) string {
+//	        // Custom getter logic
+//	        return ""
+//	    },
+//	    Unsetter: func(cfg *Config) {
+//	        // Custom unsetter logic
+//	    },
+//	})
+//
+// # Validation Helpers
+//
+// The package provides common validation functions:
+//   - validateFilePath: Validates file exists and returns cleaned path
+//   - validateFileExists: Checks if file exists
+//   - validateJSONFile: Validates file is JSON format
+//   - validateURLScheme: Validates URL scheme (http/https)
+//   - makeAbsolutePath: Converts relative to absolute path
+//
+// Use these in your validator function for consistent error messages.
+//
+// # Built-in Fields
+//
+// The following fields are currently registered:
+//   - ca-cert: Path to a CA certificate file for TLS validation
+//   - registry-url: URL of the MCP server registry (HTTP/HTTPS)
+//   - registry-file: Path to a local JSON file containing the registry
+//   - otel-endpoint: OpenTelemetry OTLP endpoint
+//   - otel-sampling-rate: Trace sampling rate (0.0-1.0)
+//   - otel-env-vars: Environment variables for telemetry
+//   - otel-metrics-enabled: Enable metrics export
+//   - otel-tracing-enabled: Enable tracing export
+//   - otel-insecure: Use insecure connection
+//   - otel-enable-prometheus-metrics-path: Enable Prometheus endpoint
+package config