[Feature Request]: Introduce a `delayed_datasets` functionality

[chlebowa]

Feature description

I would like to be able to specify a delayed_data_extract_spec for all datasets contained in the teal_data object, before I have access to the teal_data. I want soemthing similar to first_choice that will resolve the datasets in the app.

1. If teal_data includes ADSL and ADAE, this

data_extract_spec(
  dataname = delayed_datasets(c("ADSL", "ADAE")),
  ...
)

should resolve to this

list(
  data_extract_spec(
    dataname = "ADSL",
    ...
  ),
  data_extract_spec(
    dataname = "ADAE,
    ...
  )
)

2. If ADSL and ADAE are the only datasets in teal_data, this should produce the same result:

data_extract_spec(
  dataname = delayed_datasets("all"),
  ...
)

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