feat: compress backups with gzip

[link2xt]

[link2xt]

I exported backup with this and got backup file of 2775587230 bytes. Unpacked it is 3326787584 bytes, so saved 15%. Not very helpful because most blobs are not compressible, but still, 500 MB is not nothing. Did not measure how much slower it is with compression, but for backup export waiting a bit is fine. For over-the-network backup transfer it seems adding compression is not worth it.

zstd with default settings saved 50 MB more, so gzip is good enough and does not add additional dependencies. We already have the same compression for IMAP COMPRESS (gzip is deflate + some header and trailer).

[adbenitez]

it seems adding compression is not worth it.

unless we allow to import over internet

[link2xt]

Let's wait until 1.50 releases are out.