improve syntax parsing and logging

e.g. config with subscription messages indented at the wrong level is accepted and no subscription messages for websocket are sent

bad config:
```
meta:
version: latest
name: websocket-messages-connector
type: websocket-source
topic: TOPIC
create_topic: false
producer:
linger: 0ms
http:
endpoint: ws://127.0.0.1:8080/websocket-echo
subscription_messages:
- 'hello 1'
- 'hello 2'
- 'hello 3'
```

good config:
```
meta:
version: latest
name: websocket-messages-connector
type: websocket-source
topic: TOPIC
create_topic: false
producer:
linger: 0ms
http:
endpoint: ws://127.0.0.1:8080/websocket-echo
websocket_config:
subscription_messages:
- 'hello 1'
```