Allow using a compressed cache for HTTP downloads

Add a new flag for flatpak_cache_http_uri() that adds Accept-Encoding: gzip
to the request, and if the result is returned compressed, stores the data
compressed. If the data result is return uncompressed, it's compressed.

Closes: #1910
Approved by: alexlarsson
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f