refactor: rename flow output_fn to on_done

[hanxiao]

• All examples/usage that using output_fn will receive a depreciation warning, it will be auto-casted to on_done
• Make the signature of on_done, on_error, on_always visible in docstring
• New flow inheritance, Flow(BaseFlow), AsyncFlow(BaseFlow)
• New client inheritance, Client(BaseClient), AsyncClient(BaseClient)

[github-actions]

Latency summary

Current PR yields:

• 🐢🐢 index QPS at 2131, delta to last 3 avg.: -10%
• 😶 query QPS at 41, delta to last 3 avg.: -1%

Breakdown

Version	Index QPS	Query QPS
current	2131	41
0.8.7	2485	40
0.8.6	2364	42
0.8.5	2332	42

Backed by latency-tracking. Further commits will update this comment.