Improve template helper functions: string/slice (#24266)

Follow #23328

The improvements:

1. The `contains` functions are covered by tests
2. The inconsistent behavior of `containGeneric` is replaced by
`StringUtils.Contains` and `SliceUtils.Contains`
3. In the future we can move more help functions into XxxUtils to
simplify the `helper.go` and reduce unnecessary global functions.

FAQ:

1. Why it's called `StringUtils.Contains` but not `strings.Contains`
like Golang?

Because our `StringUtils` is not Golang's `strings` package. There will
be our own string functions.

---------

Co-authored-by: silverwind <me@silverwind.io>

modules/templates/util_test.go

@@ -4,6 +4,9 @@
 package templates
 
 import (
+	"html/template"
+	"io"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -41,3 +44,36 @@ func TestDict(t *testing.T) {
 		assert.Error(t, err)
 	}
 }
+
+func TestUtils(t *testing.T) {
+	execTmpl := func(code string, data any) string {
+		tmpl := template.New("test")
+		tmpl.Funcs(template.FuncMap{"SliceUtils": NewSliceUtils, "StringUtils": NewStringUtils})
+		template.Must(tmpl.Parse(code))
+		w := &strings.Builder{}
+		assert.NoError(t, tmpl.Execute(w, data))
+		return w.String()
+	}
+
+	actual := execTmpl("{{SliceUtils.Contains .Slice .Value}}", map[string]any{"Slice": []string{"a", "b"}, "Value": "a"})
+	assert.Equal(t, "true", actual)
+
+	actual = execTmpl("{{SliceUtils.Contains .Slice .Value}}", map[string]any{"Slice": []string{"a", "b"}, "Value": "x"})
+	assert.Equal(t, "false", actual)
+
+	actual = execTmpl("{{SliceUtils.Contains .Slice .Value}}", map[string]any{"Slice": []int64{1, 2}, "Value": int64(2)})
+	assert.Equal(t, "true", actual)
+
+	actual = execTmpl("{{StringUtils.Contains .String .Value}}", map[string]any{"String": "abc", "Value": "b"})
+	assert.Equal(t, "true", actual)
+
+	actual = execTmpl("{{StringUtils.Contains .String .Value}}", map[string]any{"String": "abc", "Value": "x"})
+	assert.Equal(t, "false", actual)
+
+	tmpl := template.New("test")
+	tmpl.Funcs(template.FuncMap{"SliceUtils": NewSliceUtils, "StringUtils": NewStringUtils})
+	template.Must(tmpl.Parse("{{SliceUtils.Contains .Slice .Value}}"))
+	// error is like this: `template: test:1:12: executing "test" at <SliceUtils.Contains>: error calling Contains: ...`
+	err := tmpl.Execute(io.Discard, map[string]any{"Slice": struct{}{}})
+	assert.ErrorContains(t, err, "invalid type, expected slice or array")
+}