Look into removing the openssl dependency for gcs

[luser]

I realized that the gcs backend wasn't enabled in --features=all, so I fixed that, but it broke the AppVeyor build because it requires OpenSSL, which is a PITA to configure on Windows, so I added an all-windows feature that includes everything but gcs for now.

We did used to install OpenSSL for the AppVeyor build but I'm not eager to go back to that.

The only usage of OpenSSL is for parsing PEM files. @cramertj tried using the pem-parser crate during the initial landing of the gcs backend, but it's not usable enough unfortunately. It looks like the pem crate might be suitable. Some simple testing with random .pem key files on my system shows that it produces the same data as the openssl code that's currently in sccache.

[luser]

This was fixed in #367