lfsapi/certs: use *config.URLConfig to do per-host config lookup

[ttaylorr]

This pull request updates all remaining http.{url}.* key lookups to use the new *config.URLConfig type.

I did a search via:

~/g/git-lfs (master) $ find . -type f | grep -v "\.\/vendor" | grep "\.go$" | xargs ack 'http\.'

and trimmed those down which yielded the changes made in this pull request. The only other http.* key lookup we have is here for the http.proxy config, but Git itself does not support per-host/url http.proxy-ing.

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/cc @git-lfs/core