Remove openssl dependency for GCS

[Jake-Shadle]

This is a slight reimplementation of #272 that I think works slightly better.

• It doesn't add a dependency on pem which seems like overkill for just parsing a key out, at least if it's only going to be used for GCS specifically. This just uses a simple string split since the key format is not complicated.
• I opened a PR here Attempt to fallback to pkcs8 if der fails Keats/jsonwebtoken#74 to enable the key to be passed "as-is" rather than converting it to DER. It looks like a v7 discussion Keats/jsonwebtoken#76 of jsonwebtoken coming, which might remove this API ambiguity, but for now pointing at the fork allows GCS to function properly, without OpenSSL.

[Jake-Shadle]

Went ahead and removed the now superfluous all-windows feature flag.

[luser]

Thanks for the patch! I'll try to take a look at this today.

[luser]

OK, I'm going to land this as-is. I don't love having crates.io patches because they complicate pushing new releases, but hopefully the jsonwebtoken API situation will get sorted out and we can switch back to an upstream release soon.