gtlscertificate: make private key properties readable

WebKit wants these private key properties to be readable in order to implement a deserialization function. Currently they are read-only because at the time GTlsCertificate was originally designed, the plan was to support PKCS#11-backed private keys: private keys that are stored on a smartcard, where the private key is completely unreadable. The design goal was to support both memory-backed and smartcard-backed private keys with the same GTlsCertificate API, abstracting away the implementation differences such that code using GTlsCertificate doesn't need to know the difference.

The original PKCS#11 implementation was never fully baked and at some point in the past I deleted it all. It has since been replaced with a new implementation, including a GTlsCertificate:private-key-pkcs11-uri property, which is readable. So the current API already exposes the differences between normal private keys and PKCS#11-backed private keys. The point of making the private-key and private-key-pem properties write-only was to avoid exposing this difference.

Do we have to make this API function readable? No, because WebKit could be just as well served if we were to expose serialize and deserialize functions instead. But WebKit needs to support serializing and deserializing the non-private portion of GTlsCertificate with older versions of GLib anyway, so we can do whatever is nicest for GLib. And I think making this property readable is nicest, since the original design reason for it to not be readable is now obsolete. The disadvantage to this approach is that it's now possible for an application to read the private-key or private-key-pem property, receive NULL, and think "this certificate must not have a private key," which would be incorrect if the private-key-pkcs11-uri property is set. That seems like a minor risk, but it should be documented.