TlsListenerConfig with buffer, or Read trait

afaict the TlsListenerConfig currently requires sending it PathBuf's, which eventually opens/reads the keys and certs, it would be nice if there were a way to pass it an `&[u8]` or more likely something implementing the `Read` trait, so you can use keys and certs that aren't stored on the filesystem.
```
TlsListener::build()
.addrs("localhost:4433")
.cert(cert_path)
.key(key_path),
```