feat(rules): replace go templates with custom variable expansion

- Replace template syntax ({{ .Request.Method }}) with $-prefixed variables ($req_method)
- Implement custom variable parser with static ($req_method, $status_code) and dynamic ($header(), $arg(), $form()) variables
- Replace templateOrStr interface with templateString struct and ExpandVars methods
- Add parser improvements for reliable quote handling
- Add new error types: ErrUnterminatedParenthesis, ErrUnexpectedVar, ErrExpectOneOrTwoArgs
- Update all tests and help text to use new variable syntax
- Add comprehensive unit and benchmark tests for variable expansion

internal/route/rules/vars_dynamic.go

@@ -0,0 +1,81 @@
+package rules
+
+import (
+	"net/http"
+	"net/url"
+	"strconv"
+)
+
+var (
+	VarHeader         = "header"
+	VarResponseHeader = "resp_header"
+	VarQuery          = "arg"
+	VarForm           = "form"
+	VarPostForm       = "postform"
+)
+
+type dynamicVarGetter func(args []string, w *ResponseModifier, req *http.Request) (string, error)
+
+var dynamicVarSubsMap = map[string]dynamicVarGetter{
+	VarHeader: func(args []string, w *ResponseModifier, req *http.Request) (string, error) {
+		key, index, err := getKeyAndIndex(args)
+		if err != nil {
+			return "", err
+		}
+		return getValueByKeyAtIndex(req.Header, key, index)
+	},
+	VarResponseHeader: func(args []string, w *ResponseModifier, req *http.Request) (string, error) {
+		key, index, err := getKeyAndIndex(args)
+		if err != nil {
+			return "", err
+		}
+		return getValueByKeyAtIndex(w.Header(), key, index)
+	},
+	VarQuery: func(args []string, w *ResponseModifier, req *http.Request) (string, error) {
+		key, index, err := getKeyAndIndex(args)
+		if err != nil {
+			return "", err
+		}
+		return getValueByKeyAtIndex(GetSharedData(w).GetQueries(req), key, index)
+	},
+	VarForm: func(args []string, w *ResponseModifier, req *http.Request) (string, error) {
+		key, index, err := getKeyAndIndex(args)
+		if err != nil {
+			return "", err
+		}
+		return getValueByKeyAtIndex(req.Form, key, index)
+	},
+	VarPostForm: func(args []string, w *ResponseModifier, req *http.Request) (string, error) {
+		key, index, err := getKeyAndIndex(args)
+		if err != nil {
+			return "", err
+		}
+		return getValueByKeyAtIndex(req.PostForm, key, index)
+	},
+}
+
+func getValueByKeyAtIndex[Values http.Header | url.Values](values Values, key string, index int) (string, error) {
+	// NOTE: do not use Header.Get or http.CanonicalHeaderKey here, respect to user input
+	if values, ok := values[key]; ok && index < len(values) {
+		return values[index], nil
+	}
+	// ignore unknown header or index out of range
+	return "", nil
+}
+
+func getKeyAndIndex(args []string) (key string, index int, err error) {
+	switch len(args) {
+	case 0:
+		return "", 0, ErrExpectNoArg
+	case 1:
+		return args[0], 0, nil
+	case 2:
+		index, err = strconv.Atoi(args[1])
+		if err != nil {
+			return "", 0, ErrInvalidArguments.Withf("invalid index %q", args[1])
+		}
+		return args[0], index, nil
+	default:
+		return "", 0, ErrExpectOneOrTwoArgs
+	}
+}