Support fetching arbitrary files via package manager.

[VisenDev]

Sometimes I've had the need to fetch a file with the zig package manager, but the package manager only supports compressed tarballs/zips and git repositories at the moment. I would be nice if we had a "simple" fetch mode where the package manager just fetched any file and stored it.

For example,

.my_arbitrary_file = .{
    .path = "raw_file+https://raw.githubusercontent.com/ziglang/zig/refs/heads/master/.mailmap",
    .hash = "HASH_HERE",
},

This fetch mode would effectively fetch the file in the same way wget or curl would fetch a file from a uri.

[xdBronch]

duplicate of #17895