Decode encoded HTTP content for extra-data

[gasinvein]

An HTTP server can compress the response body for transfer (setting Content-Encoding header). When downloading a file, Flatpak ignores this encoding and stores the HTTP response body as is.
The problem with this is that content encoding isn't deterministic. HTTP server may decide on its own to compress the file or not, use different compression methods, set different compression levels, set non-zero timestamp in gzip header, etc.
This can result in extra-data size and checksum being different for different clients, depending on the server decision.

Proposed solution: decode encoded content automatically when downloading extra-data and apply size/checksum checks to the decoded content. This will make the checks deterministic, regardless of Content-Encoding setting.

Related issue flathub-infra/flatpak-external-data-checker#159