[24.2] Fix default value handling for parameters connected to required parameters

[mvdbeek]

and allow authors to set a default value for optional and non-optional parameters.

59dc2a4 fails tests prior to this PR, the fix for that is in 8546b50 which is why I think we should add this to 24.2.

For optional and non-optional parameters we fill the run form with the default value and allow users to change it. The run form will prevent submitting the workflow for a non-optional value if the user has removed the default value, while for an optional parameter the workflow will be submitted with null as input. That aligns with optional input parameter outputs only being connectable to optional inputs.

With this PR we align with https://www.commonwl.org/v1.2/Workflow.html#WorkflowInputParameter:

The default value to use for this parameter if the parameter is missing from the input object, or if the value of the parameter in the input object is null.

See default_values.gxwf-tests.yml for a test case exercising this

How to test the changes?

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• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
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  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

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