add new functionality and improve backend server structure

- Enhance server functionality and optimize backend code
- Add multi-database support (PostgreSQL, MySQL, SQLite)

server/core_backend-server/tools/picker/modifiers.go

@@ -0,0 +1,41 @@
+package picker
+
+import (
+	"fmt"
+
+	"github.com/pocketbase/pocketbase/tools/tokenizer"
+)
+
+var Modifiers = map[string]ModifierFactoryFunc{}
+
+type ModifierFactoryFunc func(args ...string) (Modifier, error)
+
+type Modifier interface {
+	// Modify executes the modifier and returns a new modified value.
+	Modify(value any) (any, error)
+}
+
+func initModifer(rawModifier string) (Modifier, error) {
+	t := tokenizer.NewFromString(rawModifier)
+	t.Separators('(', ')', ',', ' ')
+	t.IgnoreParenthesis(true)
+
+	parts, err := t.ScanAll()
+	if err != nil {
+		return nil, err
+	}
+
+	if len(parts) == 0 {
+		return nil, fmt.Errorf("invalid or empty modifier expression %q", rawModifier)
+	}
+
+	name := parts[0]
+	args := parts[1:]
+
+	factory, ok := Modifiers[name]
+	if !ok {
+		return nil, fmt.Errorf("missing or invalid modifier %q", name)
+	}
+
+	return factory(args...)
+}