Use-after-free of caller-owned server name in client connection constructors

Summary

In rustls-ffi 0.12.0 through 0.15.6, `rustls_client_connection_new()` and `rustls_client_connection_new_alpn()` retained a reference to the caller's `server_name` buffer after returning, rather than copying it. The API documentation did not require the buffer to outlive the call and as a result if the caller freed or reused the buffer, later TLS processing read freed memory.

Clients that free or reuse the `server_name` buffer after the constructor returns are affected. This is most likely for language bindings that pass temporary strings. Callers that keep the `server_name` buffer alive for the lifetime of the `rustls_client_config` are not affected.

Versions 0.12.0 through 0.15.6 are affected. Version 0.15.7+ changed these APIs to copy the input buffer.