Refactoring of `hulse-py` client

[sachalevy]

Minor Refactoring

As pointed out by @ilaffey2, some functions are currently misplaced/hard to understand. Notably:

• merge the query client and the utils post_query functions
• move the get_auth_headers to the utils file
• simplify api_key flow in the client by only leaving one entry point + setters/getters
• add sample response in docs/docstring of process_stream_data