update cadf spec to support new data model

- add Credential type and move token into it
- add Host type and move client_addr, agent into it
- add Endpoint type and move adminUrl, privateUrl into it
- add credential, host, addresses as optional attr of Resource
- add observer attr to event

Change-Id: I663eaf0b2f239300e035f084b8574e615c40a182
Fixes: Bug #1214097