Icu 5651 create spk type (#1266)

Add functionality for creating a ssh private key credential and add
corresponding acceptance tests.

✅ Closes: ICU-5651

Co-authored-by: Cameron Perera <cameron.perera@hashicorp.com>
Co-authored-by: Randall Morey <randall@randallmorey.com>
Co-authored-by: Zhihe Li <zhihe.li@hashicorp.com>

addons/api/tests/unit/serializers/credential-test.js

@@ -86,6 +86,9 @@ module('Unit | Serializer | credential', function (hooks) {
           type: 'username_password',
           username: 'username',
           version: 1,
+          passphrase: '',
+          private_key: '',
+          password: '',
         },
         type: 'credential',
         id: 'credup_123',
@@ -117,6 +120,9 @@ module('Unit | Serializer | credential', function (hooks) {
           type: 'ssh_private_key',
           username: 'username',
           version: 1,
+          passphrase: '',
+          private_key: '',
+          password: '',
         },
         type: 'credential',
         id: 'credspk_123',