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/models/jobs.py

@@ -156,7 +156,7 @@ class JobOptions(BaseModel):
 
     def clean_credential(self):
         cred = self.credential
-        if cred and cred.kind != 'ssh':
+        if cred and cred.kind != 'machine':
             raise ValidationError(
                 _('You must provide a machine / SSH credential.'),
             )