Add example of dynamic directive with input file

[bentsherman]

See #4493 (comment)

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.

[bentsherman]

@christopher-hakkaart for your consideration. It would be good to have an example of setting the memory based on the task inputs, but ideally would be something that is proven to work in the wild. Instead of using data.size(), which I just made up, you might find a more concrete example in the talk that Harshil linked.

Even something like this could be better:

// allocate 1 GB for every million reads, up to 32 GB
memory { meta.num_reads / 1_000_000 * 1.GB }

[bentsherman]

@christopher-hakkaart can you give this one a review when you have a chance

[christopher-hakkaart]

Minor suggestions.

[christopher-hakkaart]

I'm happy with the language. The 1e9 notation could be removed or a note could be added to explain. It's not blocking for me.