Refactor query reading from file, update docs

Add support for @file syntax in more places.

cmd/string_or_file_flag.go

@@ -0,0 +1,55 @@
+package cmd
+
+import (
+	"io"
+	"os"
+	"strings"
+
+	"github.com/smira/flag"
+)
+
+// StringOrFileFlag is a custom flag type that can handle both string input and file input.
+// If the input starts with '@', it is treated as a filename and the contents are read from the file.
+// If the input is '@-', the contents are read from stdin.
+type StringOrFileFlag struct {
+	value string
+}
+
+func (s *StringOrFileFlag) String() string {
+	return s.value
+}
+
+func (s *StringOrFileFlag) Set(value string) error {
+	var err error
+	s.value, err = GetStringOrFileContent(value)
+	return err
+}
+
+func (s *StringOrFileFlag) Get() any {
+	return s.value
+}
+
+func AddStringOrFileFlag(flagSet *flag.FlagSet, name string, value string, usage string) *StringOrFileFlag {
+	result := &StringOrFileFlag{value: value}
+	flagSet.Var(result, name, usage)
+	return result
+}
+
+func GetStringOrFileContent(value string) (string, error) {
+	if !strings.HasPrefix(value, "@") {
+		return value, nil
+	}
+
+	filename := strings.TrimPrefix(value, "@")
+	var data []byte
+	var err error
+	if filename == "-" { // Read from stdin
+		data, err = io.ReadAll(os.Stdin)
+	} else {
+		data, err = os.ReadFile(filename)
+	}
+	if err != nil {
+		return "", err
+	}
+	return string(data), nil
+}