feat: testutil generator enhancements

[rvagg]

Also correct Tsize for sharded file storage (done elsewhere already)

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Parking this here for now; need to finish this up properly but I'm busy with byte range stuff in here in the meantime. I'll likely pull the tsize fix out to a separate PR with some tests.

[rvagg]

Pushed a big change here in feat: new UnixFS{File,Directory} with options pattern and in there I've flagged future deprecation of the Generate* functions. I finally outgrew these and want to do so much more with them for generating test data. The options pattern gives them room to grow.

WithChildGenerator is the weirdest option, but I can explain that when I push the project I've been working on. Will leave this open till then but this is all working nicely as it is for now.

[rvagg]

https://github.com/ipld/go-fixtureplate is what these changes are inspired by. The changes here expose a lot more optionality in being able to shape the output data.

One thing I'd like to add (not now) is being able to control sharding structure of large files, that's currently hidden and not very nicely configurable in the file builder in this library.