UI: allow arbitrary parameter input similar to CLI

[agilgur5]

Summary

Folllow-up to #12923 (comment).

The CLI allows you to add arbitrary parameters when submitting a WorkflowTemplate, while the UI only lets you use parameters that are already specified inside of the template.

Use Cases

When you want to add a parameter in the UI that isn't explicitly specified in the WorkflowTemplate's parameters. For instance, one that has no default or if you have some indirection etc.

Implementation Details

Probably a small button at the bottom of the parameter input that says "Add a parameter" which lets you specify a name for the parameter. Then can add a value the same as existing parameters

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