feat: extract specific path, accept stdin as streaming input

[rvagg]

(WIP: needs better testing, it works with the very limited test data I have at hand)

• Adds a --path (-p) to extract just a specific unixfs path (and its children)
• Adds - as a possible input for --file (-f) to read from stdin - the catch here is that it reads it into memory, so a large CAR is going to be a problem; but it will at least extract as the CAR blocks come in.

So cat foo.car | car extract -f - -p /my/specific/file.txt is a thing .. which will be even neater with lassie fetch <cid>/my/specific/file.txt -o -; when I make that a thing.

I think I might have also fixed the macos-12 flaky test problem, maybe it's not a github.com/rogpeppe/go-internal problem after all but related to how macos-12 handles buffered file reads.

[rvagg]

Latest commit adds - as an argument option that will extract a file to stdout. It'll error if it has to do anything more than a single file.

An alternative would be to provide --stdout as an argument rather than - since the output path is optional already and - is a valid (although very odd) output path that someone make want.

cat foo.car | car extract -f - -p /my/specific/file.txt - - now spits the contents of file.txt to stdout.

[willscott]

this looks good! let me know when you want a full read through the code

[rvagg]

Highlighting this while I think about it and so it doesn't get lost in the code move—in the case where you want to extract all, rather than a specific path, instead of erroring where a block load fails, it skips whatever it's failed to load. This needs more testing, and perhaps it's not feasable, because I've got an example where this partially works but then fails with an iterator overrun error which I don't yet understand. I'm guessing it's to do with sharded directory incompleteness but I need to work that out.

[rvagg]

Added a big 'ol test script and some fixture data; ready for review @willscott