Support compression for Actions logs

[wolfogre]

Support compression for Actions logs to save storage space and bandwidth. Inspired by #24256 (comment)

The biggest challenge is that the compression format should support seekable. So when users are viewing a part of the log lines, Gitea doesn't need to download the whole compressed file and decompress it.

That means gzip cannot help here. And I did research, there aren't too many choices, like bgzip and xz, but I think zstd is the most popular one. It has an implementation in Golang with zstd and zstd-seekable-format-go, and what is better is that it has good compatibility: a seekable format zstd file can be read by a regular zstd reader.

This PR introduces a new package zstd to combine and wrap the two packages, to provide a unified and easy-to-use API.

And a new setting LOG_COMPRESSION is added to the config, although I don't see any reason why not to use compression, I think's it's a good idea to keep the default with none to be consistent with old versions.

LOG_COMPRESSION takes effect for only new log files, it adds .zst as an extension to the file name, so Gitea can determine if it needs decompression according to the file name when reading. Old files will keep the format since it's not worth converting them, as they will be cleared after #31735.

[silverwind]

And a new setting LOG_COMPRESSION is added to the config, although I don't see any reason why not to use compression, I think's it's a good idea to keep the default with none to be consistent with old versions.

Why not enable compression by default? Can the log reading code handle a mix of compressed and uncompressed data? If so, I see no reason for not defaulting to it.

[wolfogre]

Can the log reading code handle a mix of compressed and uncompressed data?

Yes, it works fine with that.

If so, I see no reason for not defaulting to it.

TBH, I also don't see a strong reason for no compression by default. I cannot think of any case where enabling compression would have a negative impact. But I have some concerns:

1. zstd-seekable-format-go is not so popular. It needs more time to prove it's stable enough. If it has critical bugs like memory leaks or deadlocks, it will have a lesser effect to Gitea when LOG_COMPRESSION is set to none by default.
2. I'm not familiar with tiny servers like Raspberry Pi, or other servers with limited memory and busy CPU, so I have no idea if compression could cause a negative impact for them.

So my idea is to introduce LOG_COMPRESSION in this PR, and set it to zstd by default in another one when we are sure it works well.

---

I opened a issue #31801 in case we forget to do that.

[wolfogre]

I almost forget to update https://gitea.com/gitea/docs.

So please help review:

1. https://gitea.com/gitea/docs/pulls/50, which updates the docs.
2. Add label docs-update-needed for PRs that modify app.example.ini #31810, in case I forget again (maybe a label can still be easily overlooked, but it's better than having none).
3. Fix typo for LOG_COMPRESSION in ini #31809, it fixes a typo in this PR, and it can be used to test the new labeler rule.