Use json-iterator for secrets marshalling (#44309)

### What does this PR do?

Use json-iterator for secrets marshalling in the file `comp/core/secrets/impl/fetch_secret.go`

### Motivation

If my `datadog.yaml config` contains this:

```
secret_backend_command: /opt/datadog-agent/scripts/script.py
secret_backend_type: aws.secrets
secret_backend_config:
  a:
    b: c
api_key: ENC[my_api_key]
```

Then running this code with `DD_CONF_NODETREEMODEL=enable` leads to `b: c` having type `map[interface{}]interface{}` which `encoding/json` fails to marshal. The third-party package `"github.com/json-iterator/go"` will correctly marshal this.

### Describe how you validated your changes

### Additional Notes

It's unclear why enabling NTM causes this different runtime type. It could be a good idea to make sure the yaml reader never returns objects of type `map[interface{}]interface{}`.

Co-authored-by: dustin.long <dustin.long@datadoghq.com>

Author: dustmop

comp/core/secrets/fx/go.mod

@@ -43,11 +43,14 @@ require (
 	github.com/gofrs/flock v0.13.0 // indirect
 	github.com/hectane/go-acl v0.0.0-20230122075934-ca0b05cb1adb // indirect
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
+	github.com/json-iterator/go v1.1.12 // indirect
 	github.com/lufia/plan9stats v0.0.0-20250317134145-8bc96cf8fc35 // indirect
 	github.com/mattn/go-colorable v0.1.14 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	github.com/mdlayher/socket v0.5.1 // indirect
 	github.com/mdlayher/vsock v1.2.1 // indirect
+	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
+	github.com/modern-go/reflect2 v1.0.3-0.20250322232337-35a7c28c31ee // indirect
 	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
 	github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2 // indirect
 	github.com/power-devops/perfstat v0.0.0-20240221224432-82ca36839d55 // indirect

comp/core/secrets/fx/go.sum

@@ -8,14 +8,15 @@ package secretsimpl
 import (
 	"bytes"
 	"context"
-	"encoding/json"
 	"errors"
 	"fmt"
 	"os/exec"
 	"strconv"
 	"strings"
 	"time"
 
+	json "github.com/json-iterator/go"
+
 	secrets "github.com/DataDog/datadog-agent/comp/core/secrets/def"
 	"github.com/DataDog/datadog-agent/pkg/util/log"
 )

comp/core/secrets/impl/fetch_secret.go

@@ -17,6 +17,7 @@ require (
 	github.com/DataDog/datadog-agent/pkg/util/testutil v0.59.0
 	github.com/DataDog/datadog-agent/pkg/util/winutil v0.68.3
 	github.com/benbjohnson/clock v1.3.5
+	github.com/json-iterator/go v1.1.12
 	github.com/stretchr/testify v1.11.1
 	golang.org/x/exp v0.0.0-20251209150349-8475f28825e9
 	golang.org/x/sys v0.39.0
@@ -52,6 +53,8 @@ require (
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	github.com/mdlayher/socket v0.5.1 // indirect
 	github.com/mdlayher/vsock v1.2.1 // indirect
+	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
+	github.com/modern-go/reflect2 v1.0.3-0.20250322232337-35a7c28c31ee // indirect
 	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
 	github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2 // indirect
 	github.com/power-devops/perfstat v0.0.0-20240221224432-82ca36839d55 // indirect