Binary caching webdav or S3 backend support

[xiaozhuai]

It would be awesome if vcpkg could support storing binary cache on a webdav or S3 server.
I considered to mount webdav or S3 to filesystem. But there would be many problems.
It will be better if vcpkg support webdav or S3 directly.

[JackBoosY]

Yeah, although vcpkg provides binarycaching functionality, we still do not currently provide precompiled binaries.