add text transform support

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

options.go

@@ -1,7 +1,12 @@
 package file
 
 import (
+	"io"
+	"os"
+	"regexp"
+
 	"go.unistack.org/micro/v3/config"
+	"golang.org/x/text/transform"
 )
 
 type pathKey struct{}
@@ -21,3 +26,39 @@ func SavePath(path string) config.SaveOption {
 func WatchPath(path string) config.WatchOption {
 	return config.SetWatchOption(pathKey{}, path)
 }
+
+type readerKey struct{}
+
+func Reader(r io.Reader) config.Option {
+	return config.SetOption(readerKey{}, r)
+}
+
+type transformerKey struct{}
+
+type TransformerFunc func(src []byte, index []int) []byte
+
+func Transformer(t transform.Transformer) config.Option {
+	return config.SetOption(transformerKey{}, t)
+}
+
+func NewEnvTransformer(rs string, trimLeft, trimRight int) (*EnvTransformer, error) {
+	re, err := regexp.Compile(rs)
+	if err != nil {
+		return nil, err
+	}
+	return &EnvTransformer{
+		Regexp: re,
+		TransformerFunc: func(src []byte, index []int) []byte {
+			var envKey string
+			if len(src) > index[1]-trimRight {
+				envKey = string(src[index[0]+trimLeft : index[1]-trimRight])
+			}
+
+			if envVal, ok := os.LookupEnv(envKey); ok {
+				return []byte(envVal)
+			}
+
+			return src[index[0]:index[1]]
+		},
+	}, nil
+}