HTTPS: Support Secure extension for HTTP

**Is your feature request related to a problem? Please describe.**
Right now there is only HTTP server without ability to use secure extension using SSL or TLS. But there is need to test clients that uses HTTPS protocol for communication.

**Describe the solution you'd like**
- Provide RobotFramework API to configure HTTP to work through SSL or TLS (enable HTTPS support).
- User should be able to specify a certificate for the server and for the client.
- User should be able to specify a private key for the server and for the client.

**Describe alternatives you've considered**
Maybe separate HTTPS server library, but it will reuse a lot of functionality from HTTP.

**Additional context**
N/A