Add DuckDuckGo support and refactor caching system

Introduced DuckDuckGo as a new search provider alongside Google. Implemented a flexible caching system with in-memory, file-based, and no-op cache options to improve modularity. Updated dependencies and revised the project structure for improved maintainability.

pkg/cache/memory.go

@@ -0,0 +1,78 @@
+package cache
+
+import (
+	"encoding/json"
+	"io"
+)
+
+type memoryCache struct {
+	data map[string][]byte
+}
+
+var _ Cache = &memoryCache{}
+
+func NewMemoryCache() (Cache, error) {
+	return &memoryCache{
+		data: make(map[string][]byte),
+	}, nil
+}
+
+func (m *memoryCache) Get(key string, writer io.Writer) error {
+	data, ok := m.data[key]
+	if ok {
+		_, err := writer.Write(data)
+		return err
+	}
+
+	return ErrNotFound
+}
+
+func (m *memoryCache) GetString(key string) (string, error) {
+	data, ok := m.data[key]
+	if ok {
+		return string(data), nil
+	}
+
+	return "", ErrNotFound
+}
+
+func (m *memoryCache) GetJSON(key string, value interface{}) error {
+	data, ok := m.data[key]
+	if ok {
+		return json.Unmarshal(data, value)
+	}
+
+	return ErrNotFound
+}
+
+func (m *memoryCache) Set(key string, value io.Reader) error {
+	data, err := io.ReadAll(value)
+	if err != nil {
+		return err
+	}
+
+	m.data[key] = data
+
+	return nil
+}
+
+func (m *memoryCache) SetJSON(key string, value interface{}) error {
+	data, err := json.Marshal(value)
+	if err != nil {
+		return err
+	}
+
+	m.data[key] = data
+
+	return nil
+}
+
+func (m *memoryCache) SetString(key string, value string) error {
+	m.data[key] = []byte(value)
+	return nil
+}
+
+func (m *memoryCache) Delete(key string) error {
+	delete(m.data, key)
+	return nil
+}

pkg/cache/nop.go

@@ -0,0 +1,38 @@
+package cache
+
+import (
+	"io"
+)
+
+type Nop struct {
+}
+
+var _ Cache = Nop{}
+
+func (Nop) Get(_ string, _ io.Writer) error {
+	return ErrNotFound
+}
+
+func (Nop) GetString(_ string) (string, error) {
+	return "", ErrNotFound
+}
+
+func (Nop) GetJSON(_ string, _ interface{}) error {
+	return ErrNotFound
+}
+
+func (Nop) Set(_ string, _ io.Reader) error {
+	return nil
+}
+
+func (Nop) SetJSON(_ string, _ interface{}) error {
+	return nil
+}
+
+func (Nop) SetString(_ string, _ string) error {
+	return nil
+}
+
+func (Nop) Delete(_ string) error {
+	return nil
+}