Introduce a new CredentialTemplate model

Credentials now have a required CredentialType, which defines inputs
(i.e., username, password) and injectors (i.e., assign the username to
SOME_ENV_VARIABLE at job runtime)

This commit only implements the model changes necessary to support the
new inputs model, and includes code for the credential serializer that
allows backwards-compatible support for /api/v1/credentials/; tasks.py
still needs to be updated to actually respect CredentialType injectors.

This change *will* break the UI for credentials (because it needs to be
updated to use the new v2 endpoint).

see: #5877
see: #5876
see: #5805

awx/main/tests/functional/api/test_job_runtime_params.py

@@ -10,8 +10,15 @@ from awx.api.versioning import reverse
 
 
 @pytest.fixture
-def runtime_data(organization):
-    cred_obj = Credential.objects.create(name='runtime-cred', kind='ssh', username='test_user2', password='pas4word2')
+def runtime_data(organization, credentialtype_ssh):
+    cred_obj = Credential.objects.create(
+        name='runtime-cred',
+        credential_type=credentialtype_ssh,
+        inputs={
+            'username': 'test_user2',
+            'password': 'pas4word2'
+        }
+    )
     inv_obj = organization.inventories.create(name="runtime-inv")
     return dict(
         extra_vars='{"job_launch_var": 4}',