fix(libutil/tarfile): normalize legacy HTTP Content-Encoding names

[lovesegfault]

Motivation

Nix failed to download files served with Content-Encoding: x-gzip
because libarchive doesn't recognize the legacy x-* compression
format names. Per RFC 9110 §8.4.1.3, HTTP recipients should treat
these as equivalent to their standard counterparts.

Adds normalizeCompressionMethod() to map legacy encoding names
before passing to libarchive:

• x-gzip → gzip
• x-compress → compress
• x-bzip2 → bzip2

Context

Fixes: #14324

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[xokdvium]

I think that what we should do is stop using strings to represent enumeration types. We confuse compression algorithm name used by libarxhive and our non-standard Content-Enxoding headers. Those need to become clearly separated

[Mic92]

I think that what we should do is stop using strings to represent enumeration types. We confuse compression algorithm name used by libarxhive and our non-standard Content-Enxoding headers. Those need to become clearly separated

you mean libarchive has enum types for compression?

[xokdvium]

libarchive has enum types for compression?

It doesn't unfortunately, but we really should have our own to wrap around libarchive.

[Ericson2314]

I agree that making our own enum sounds like the right call.