add and link libcurl

[Neumann-A]

Hmmm somebody said they need vcpkg to link libcurl directly instead of using curl. So here is an example how to implement that. (#1043)

[Neumann-A]

Don't quite understand why recv is unavailable in linux CI.
Nevermind -Werror broke all the stuff

[BillyONeal]

I think assuming libcurl.so will work is 'safe' on *nix?

My main concern with this is accidentally embedding 'which OpenSSL are you using' assumptions into our binaries, but we should be able to use the system's bundled libcurl.so as a firewall for this.

[BillyONeal]

(That is to say, on Windows, static link libcurl and use schannel backend, and rip out all the WinHTTP stuff. On non-Windows, use the system's libcurl.so. Then we can statically link the real Azure Storage SDKs to get sane uploads of >5TB. Does that seem reasonable to you?)

[Neumann-A]

@BillyONeal: Feel free to edit. I just made this PR to enable others who want to explorer this way. Probably just replace option(VCPKG_DEPENDENCY_EXTERNAL_LIBCURL "Use an external version of the libcurl library" OFF) with a platform dependent ON/OFF. use schannel backend don't know the option for it but it is probably in the portfile of curl somewhere.

Does that seem reasonable to you?

hmm maybe? I honestly have no clue what you are talking about. As I said, I just made this PR to enable others.