missing private key method in the SslContext

[KfreeZ]

the boring-sys has the c-bindings for "SSL_CTX_set_private_key_method" and "SSL_set_private_key_method" but missing in the SslContextBuilder "https://github.com/cloudflare/boring/blob/master/boring/src/ssl/mod.rs"

Is this by design of just a missing implementation? These private key method is used when we want to delegate the key operations, I'd like to help to add these if they are needed.

[nox]

I have written bindings for those in #134.