Implement SSL Support

[sylvrs]

Overview

Before this pull request, libasynql lacked the ability to connect to a MySQL server using an SSL connection (as seen by #80). This pull request aims to resolve that by adding a simple wrapper using an SSL credentials class (MysqlSslCredentials) and matching configuration options. These configuration options can be used like so:

database:
  type: mysql
  mysql:
    # ...
    ssl:
      key: ""
      certificate: ""
      ca-certificate: ""
      ca-path: ""
      cipher-algorithms: ""

Testing

Using two instances, one with SSL and one without SSL, I was able to confirm that the functionality remains the same. Furthermore, errors are properly thrown if an SSL instance is not given the proper credentials.

Backwards Compatibility

Compatibility for the library remains the same as the SSL credentials are nullable and prefilled with a null value for both fromArray and the constructor.

[SOF3]

These should be nullable. Not familiar with mysqli, but why do we need both caCertificate and caPath? What is the different between them? The typical CA bundle only involves key + cert + CA afaik.

[sylvrs]

This is just a loose binding to mysqli's ssl_set function. I can remove those parameters if you'd prefer? Regardless, I will make them nullable.

[SOF3]

Please update the documentation too so that users know this feature exists. Or if you think it is too distractive for users, at least document somewhere else, in particular e.g. whether the certificates are paths or binary bytes or what.