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

pkg/madmin/generic-commands.go

@@ -28,16 +28,16 @@ type SetCredsReq struct {
 	SecretKey string `json:"secretKey"`
 }
 
-// SetCredentials - Call Set Credentials API to set new access and
+// SetAdminCredentials - Call Set Credentials API to set new access and
 // secret keys in the specified Minio server
-func (adm *AdminClient) SetCredentials(access, secret string) error {
+func (adm *AdminClient) SetAdminCredentials(access, secret string) error {
 	// Setup request's body
 	body, err := json.Marshal(SetCredsReq{access, secret})
 	if err != nil {
 		return err
 	}
 
-	ebody, err := EncryptServerConfigData(adm.secretAccessKey, body)
+	ebody, err := EncryptData(adm.secretAccessKey, body)
 	if err != nil {
 		return err
 	}