Add actual inline data to JSON output in xl-meta (#19958)

Add the inlined data as base64 encoded field and try to add a string version if feasible.

Example:

```
λ xl-meta -data xl.meta
{
  "8e03504e-1123-4957-b272-7bc53eda0d55": {
    "bitrot_valid": true,
    "bytes": 58,
    "data_base64": "Z29sYW5nLm9yZy94L3N5cyB2MC4xNS4wIC8=",
    "data_string": "golang.org/x/sys v0.15.0 /"
}
```

The string will have quotes, newlines escaped to produce valid JSON.

If content isn't valid utf8 or the encoding otherwise fails, only the base64 data will be added.

`-export` can still be used separately to extract the data as files (including bitrot).

docs/debugging/xl-meta/main.go

@@ -20,6 +20,7 @@ package main
 import (
 	"bytes"
 	"crypto/md5"
+	"encoding/base64"
 	"encoding/binary"
 	"encoding/hex"
 	"encoding/json"
@@ -34,6 +35,7 @@ import (
 	"strconv"
 	"strings"
 	"time"
+	"unicode/utf8"
 
 	"github.com/google/uuid"
 	"github.com/klauspost/compress/zip"
@@ -237,7 +239,7 @@ FLAGS:
 			}
 
 			if c.Bool("data") {
-				b, err := data.json()
+				b, err := data.json(true)
 				if err != nil {
 					return nil, err
 				}
@@ -562,7 +564,7 @@ func (x xlMetaInlineData) versionOK() bool {
 	return x[0] > 0 && x[0] <= xlMetaInlineDataVer
 }
 
-func (x xlMetaInlineData) json() ([]byte, error) {
+func (x xlMetaInlineData) json(value bool) ([]byte, error) {
 	if len(x) == 0 {
 		return []byte("{}"), nil
 	}
@@ -607,6 +609,17 @@ func (x xlMetaInlineData) json() ([]byte, error) {
 			} else {
 				s += ", \"bitrot_valid\": false"
 			}
+			if value {
+				if utf8.Valid(data) {
+					// Encode as JSON string.
+					b, err := json.Marshal(string(data))
+					if err == nil {
+						s += `, "data_string": ` + string(b)
+					}
+				}
+				// Base64 encode.
+				s += `, "data_base64": "` + base64.StdEncoding.EncodeToString(data) + `"`
+			}
 			s += "}"
 		}
 		res = append(res, []byte(s)...)