Users should be able to define their own schema namespace

Currently, users have their namespace automatically created for them at `python.kafka.modelname`

https://github.com/AbsaOSS/py2k/blob/14d82e5ebc0c8ef6acbf431834f6585de22b3863/py2k/record.py#L66-L80

We should allow users to define their own namespace.

something like,

```python
class MyRecord(KafkaRecord):
name: str

@property
def namespace():
return 'my.name.space'
````

or as an argument to `PandasToRecordsTransformer`