chore: 🤖 JSON credential data modeling (#1485)

* chore: 🤖 JSON credential data modeling

Added json type to credential model, created `json_object` attribute for
`json` type, updated credential serializer to handle `json_object`
attribute data being sent to backend and updated normalizer to handlle
secret untracked 'json_object'.

✅ Closes: ICU-6653, ICU-6684, ICU-6685, ICU-6802, ICU-6803

* Update en-us.yaml

fixed spelling

* Update addons/api/addon/models/credential.js

Co-authored-by: Cameron Perera <cameron.perera@hashicorp.com>

* refactor: 💡 refactor credential serializer and .yaml

Refactored credential serializer to check for `json_object` existence
before serializing and changed help variable in `.yaml` file for
credentials.

* refactor: 💡 refactor for credential serializer

Refactor credential serializer removed redundant code,

Co-authored-by: Cameron Perera <cameron.perera@hashicorp.com>

addons/api/addon/generated/models/credential.js

@@ -79,4 +79,12 @@ export default class GeneratedCredentialModel extends BaseModel {
     description: 'The private key passphrase for credential.',
   })
   private_key_passphrase;
+
+  @attr('string', {
+    for: 'json',
+    isNestedAttribute: true,
+    isSecret: true,
+    description: 'The secret associated with credential.',
+  })
+  json_object;
 }