Static embed files should not be decompress and compress

[lunny]

As I said on #7080 (comment) , currently we use http.FileSystem to open a embed file on memory, that will decompress from a gzip compressed memory file and then we serve it as a http request and compress it(maybe). So I think that we can provide the gzip compressed file directly.

From vfsgen, we could detect if the File is httpgzip.NotWorthGzipCompressing or httpgzip.GzipByter to do that.

[stale]

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