Allow top-level fields in node declaration

[phil-opp]

We plan to deprecate operators in their current form in a future release. This PR implements the proposed new syntax for declaring custom nodes without an extra level of nesting:

nodes:
    - id: node_1
      path: something.py
      inputs:
        - input_1: "image"
        - input_2: "audio"
      outputs:
        - output_1
        - output_2

The inputs/outputs/etc fields are now directly declared next to the id field, instead of being nested under a custom field. The source field was renamed to path as discussed in our last meeting.

TODO:

• add path alias to custom->source field to allow both names.

[haixuanTao]

Could you actually run

cargo run -p dora-core generate_schemas

To regenerate the JSON Schema.

Maybe I should have put a script to do this automatically

[phil-opp]

I regenerated the schemas in 7364860.

[haixuanTao]

LGTM