would make sense to support CLI argument for an input with two types: `File` and `string`

Hello!

we have a case when an input to a `CommandLineTool` can be `File` or `string`. A minimal example is here:

https://github.com/common-workflow-language/cwltool/compare/main...volodymyrss:cwltool:main

However, this does not allow to make a cwltool CLI argument, it seems intentionally:

https://github.com/common-workflow-language/cwltool/blob/main/cwltool/argparser.py#L965

Would it make sense that this particular case of two arguments would also be supported? We could try to help with implementation if you think it makes sense.

Thank you!

Volodymyr

/cc @maxnoe