feat(packer): add compression support for extrafiles

[damdo]

GitHub has a hard limit of 100MB for extra files.
If these files are mostly text based (e.g. code), a quick win to circumvent this is to compress/decompress them.
This PR adds support for gzip compressed archives.

[damdo]

cc. @stapelberg

[damdo]

Closing/reopening to kick off testing again

[stapelberg]

Why not zstd? It’ll eventually be in the standard library, if that’s the reason you reached for gzip :) golang/go#62513

[damdo]

Yeah I thought about zstd, I went with gzip for 2 reasons:

1. as you guessed I used compress/gzip due to it being in the standard library (but it sounds like it'll be eventually, so this point falls)
2. contrary to most linux distros which use GNU tar, there is no way to do zstd compression with the default tooling available on macOS with BSD tar

I am in favour of dropping gzip in favour of zstd (for mac users it'll require brew install zstd to be in place). It shouldn't be too much of a hassle for them if they ever wanted to do that.

[damdo]

Ok just pushed an update to use zstd.
I've tested it with https://github.com/damdo/gokrazy-go/tree/main/_gokrazy
Works as expected.

[damdo]

Ready for a review :)