is_binary and is_structured should be in http module

## Expected Behavior
```python
from cloudevents.http import is_binary, is_structured
```

## Actual Behavior
```python
from cloudevents.sdk.converters import is_binary, is_structured
```

These methods at the moment comform to the http spec but aren't generalized to other specs. Therefore we would want to move these methods into the http module

