[FEATURE] Make cocoindex APIs friendly for Python REPL

[badmonster0]

Problems:

• Flow definition happens in a structured function. Users cannot do it step-by-step if they want.
• Setup is mainly done by CLI now, and API not smooth. Also setup and update have to happen in separate processes for now.

TODOs:

• Allow users to create flow builder and build flows by manually creating a FlowBuilder without defining a function using the decorator.
• Flow objects should display something meaningful (e.g. schema / scope for a DataSlice) when users show it.
• Make setup API well supported.
• Properly handle setup-related states, so that after setup done users can do update in the same process.

After solving the problems, both Python REPL and Colab can be more interactive in performing mini steps and showing intermediate results.