Use caCertificatePemFile in OpenSSL backend

[sebihoefle]

This PR considers the caCertificatePemFile for the OpenSSL backend, allowing to accept self-signed certificates, useful for local development (https://letsencrypt.org/docs/certificates-for-localhost/#for-local-development).

Expected:
Self-signed certificates should be accepted if the client adds the certificate as verified.

Example:

Creation of certificates:

openssl req -x509 -out localhost.crt -keyout localhost.key \
  -newkey rsa:2048 -nodes -sha256 \
  -subj '/CN=localhost' -extensions EXT -config <( \
   printf "[dn]\nCN=localhost\n[req]\ndistinguished_name = dn\n[EXT]\nsubjectAltName=DNS:localhost\nkeyUsage=digitalSignature\nextendedKeyUsage=serverAuth")

Server side:

  WebSocketServer::Configuration serverConfig;
  serverConfig.port = 48080;
  serverConfig.enableTls = true;
  serverConfig.certificatePemFile = "localhost.crt";
  serverConfig.keyPemFile = "localhost.key";
  serverConfig.bindAddress = "localhost";

Client:

  WebSocket::Configuration config;
  config.disableTlsVerification = false;
  config.caCertificatePemFile = "localhost.crt";
  WebSocket ws(config);

[sebihoefle]

Thanks @paullouisageneau , wasn't aware of SSL_CTX_load_verify_locations in [email protected].