feat: option to use custom curl bin

[kawre]

ref: https://github.com/kawre/leetcode.nvim/issues/44

TLDR: if you use msys curl on windows, and use -d with a path to a payload file, msys will not make the command execute unless you wrap the payload with '. I guess the simplest solution to this is to have an option to specify custom curl bin path, so if the user still wishes to use msys as the default curl, they still have an option to specify for example cygwin as the curl that plenary.nvim will use.