Add a TYPE and defaults properties to the flow output

[anna-geller]

Feature description

As a follow-up to those issues:

• Allow subflows to generate and pass outputs in the same way as functions can return data #2133
• Allow overriding the default subflow output behavior to avoid a breaking change #2952

We want to align flow outputs with inputs, so it would be great to add the TYPE and defaults properties.

[anna-geller]

closing as it seems that defaults is not necessary for outputs and type has been added