TLS certificate strings and custom SNI validation

[j4k0xb]

Please consider adding these features:

1. Allow setting the trust_store etc. to a string that contains the certificate, instead a path. Use case: crate ships with custom CA certificate of the server, avoid creating temporary files

2. Allow passing a custom server name that will be used for validating the SNI field, instead of using the host name. Use case: host name of the connection (IP) is different than the certificate's SNI (device serial number). See Node.js TLS servername parameter and paho.mqtt.c for how this could be implemented.