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/config-migrate_test.go

@@ -159,8 +159,8 @@ func TestServerConfigMigrateInexistentConfig(t *testing.T) {
 	}
 }
 
-// Test if a config migration from v2 to v29 is successfully done
-func TestServerConfigMigrateV2toV29(t *testing.T) {
+// Test if a config migration from v2 to v30 is successfully done
+func TestServerConfigMigrateV2toV30(t *testing.T) {
 	rootPath, err := ioutil.TempDir(globalTestTmpDir, "minio-")
 	if err != nil {
 		t.Fatal(err)
@@ -222,6 +222,7 @@ func TestServerConfigMigrateV2toV29(t *testing.T) {
 	if globalServerConfig.Credential.AccessKey != accessKey {
 		t.Fatalf("Access key lost during migration, expected: %v, found:%v", accessKey, globalServerConfig.Credential.AccessKey)
 	}
+
 	if globalServerConfig.Credential.SecretKey != secretKey {
 		t.Fatalf("Secret key lost during migration, expected: %v, found: %v", secretKey, globalServerConfig.Credential.SecretKey)
 	}