Add the dataset_expression as part of DagModel and DAGDetailSchema

[sunank200]

• Add the dataset_expression as part of DagModel and DAGDetailSchema

For example, the following dataset expression: (dag1_dataset | (dag2_dataset & dag3_dataset)) is returned as part of dictionary.

"dataset_expression": {
    "any": [
      "s3://dag1/output_1.txt",
      {
        "all": [
          "s3://dag2/output_1.txt",
          "s3://dag3/output_3.txt"
        ]
      }
    ]
  }

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[uranusjr]

I kind of wonder if this should cover the timetable as well i.e. have schedule_expression instead of special-casing datasets. We can maybe also put this on DagModel directly.

[sunank200]

@uranusjr as per conversation on slack I have done the following change:

But I feel this should not be done in the schema at all, instead this JSON should be generated when a DagModel is created (before serialization happens), and the schema should just need to read the field out