Private key parsing fails with zerossl header
zerossl provides files like this:
```
-----BEGIN RSA PRIVATE KEY-----
[the private key]
-----END RSA PRIVATE KEY-----
```

But at parsing, we only support `BEGIN PRIVATE KEY`:
https://github.com/status-im/nim-chronos/blob/9df76c39df254c7ff0cec6dec5c9f345f2819c91/chronos/streams/tlsstream.nim#L698