Type notation of Getter.get should update.

Currently 
type notation of get method of Getter class in getter.py is

<pre>
 def get(self, key: str) -> Union[str, None]:
</pre>
.

But it is incorrect and type checkers in IDE warn.

Correct type notation is as follows:
 <pre>
def get(self, key: Union[bytes, str, list, dict]) -> AgentResponse:
</pre>
.