Introduce STS client grants API and OPA policy integration (#6168)

This PR introduces two new features

- AWS STS compatible STS API named AssumeRoleWithClientGrants

```
POST /?Action=AssumeRoleWithClientGrants&Token=<jwt>
```

This API endpoint returns temporary access credentials, access
tokens signature types supported by this API

  - RSA keys
  - ECDSA keys

Fetches the required public key from the JWKS endpoints, provides
them as rsa or ecdsa public keys.

- External policy engine support, in this case OPA policy engine

- Credentials are stored on disks

cmd/fs-v1.go

@@ -1018,7 +1018,7 @@ func (fs *FSObjects) listDirFactory(isLeaf isLeafFunc) listDirFunc {
 	listDir := func(bucket, prefixDir, prefixEntry string) (entries []string, delayIsLeaf bool) {
 		var err error
 		entries, err = readDir(pathJoin(fs.fsPath, bucket, prefixDir))
-		if err != nil {
+		if err != nil && err != errFileNotFound {
 			logger.LogIf(context.Background(), err)
 			return
 		}
@@ -1274,7 +1274,7 @@ func (fs *FSObjects) ListBucketsHeal(ctx context.Context) ([]BucketInfo, error)
 
 // SetBucketPolicy sets policy on bucket
 func (fs *FSObjects) SetBucketPolicy(ctx context.Context, bucket string, policy *policy.Policy) error {
-	return savePolicyConfig(fs, bucket, policy)
+	return savePolicyConfig(ctx, fs, bucket, policy)
 }
 
 // GetBucketPolicy will get policy on bucket