Allow disabling bzip2 compression for index files

Using bzip2 generates smaller index files (roughly 20% smaller Packages
files) but it comes with a big performance penalty.  When publishing a
debian mirror snapshot (amd64, arm64, armhf, source) without contents
skipping bzip speeds things up around 1.8 times.

```
$ hyperfine -w 1 -L skip-bz2 true,false  -m 3 -p "aptly -config aptly.conf publish drop bullseye || true" "aptly -config aptly.conf  publish snapshot  --skip-bz2={skip-bz2} --skip-contents --skip-signing bullseye"
Benchmark 1: aptly -config aptly.conf  publish snapshot  --skip-bz2=true --skip-contents --skip-signing bullseye
  Time (mean ± σ):     35.567 s ±  0.307 s    [User: 39.366 s, System: 10.075 s]
  Range (min … max):   35.311 s … 35.907 s    3 runs

Benchmark 2: aptly -config aptly.conf  publish snapshot  --skip-bz2=false --skip-contents --skip-signing bullseye
  Time (mean ± σ):     64.740 s ±  0.135 s    [User: 68.565 s, System: 10.129 s]
  Range (min … max):   64.596 s … 64.862 s    3 runs

Summary
  'aptly -config aptly.conf  publish snapshot  --skip-bz2=true --skip-contents --skip-signing bullseye' ran
    1.82 ± 0.02 times faster than 'aptly -config aptly.conf  publish snapshot  --skip-bz2=false --skip-contents --skip-signing bullseye'
```

Allow skipping bz2 creation for setups where faster publishing is more
important then Package file size.

Signed-off-by: Sjoerd Simons <sjoerd@collabora.com>
Upstream MR: https://github.com/aptly-dev/aptly/pull/1081