Add example of dynamic directive with input file

[bentsherman]

See https://github.com/nextflow-io/nextflow/pull/4493#discussion_r1397589519

I don't really like this particular example because it is probably an anti-pattern, i.e. many tools are designed to stream the input data so that they don't have to load entire datasets in memory. But I want to explicitly show that input files can also be used to configure directives.

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[drpatelh]

There are some other real-world examples in this talk: https://youtu.be/oAdSpdUrJag?t=16928

Would be awesome to have something like this to tune resources.