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/api-errors.go

@@ -192,6 +192,7 @@ const (
 	ErrAdminConfigNoQuorum
 	ErrAdminConfigTooLarge
 	ErrAdminConfigBadJSON
+	ErrAdminConfigDuplicateKeys
 	ErrAdminCredentialsMismatch
 	ErrInsecureClientRequest
 	ErrObjectTampered
@@ -881,11 +882,16 @@ var errorCodeResponse = map[APIErrorCode]APIError{
 	ErrAdminConfigTooLarge: {
 		Code: "XMinioAdminConfigTooLarge",
 		Description: fmt.Sprintf("Configuration data provided exceeds the allowed maximum of %d bytes",
-			maxConfigJSONSize),
+			maxEConfigJSONSize),
 		HTTPStatusCode: http.StatusBadRequest,
 	},
 	ErrAdminConfigBadJSON: {
 		Code:           "XMinioAdminConfigBadJSON",
+		Description:    "JSON configuration provided is of incorrect format",
+		HTTPStatusCode: http.StatusBadRequest,
+	},
+	ErrAdminConfigDuplicateKeys: {
+		Code:           "XMinioAdminConfigDuplicateKeys",
 		Description:    "JSON configuration provided has objects with duplicate keys",
 		HTTPStatusCode: http.StatusBadRequest,
 	},