Type-safe Security Configurations

> Adds some of the missing properties from swift-server#46.

Motivation:

We were lacking some of the security protocol configuration options and
want to provide them in a type-safe manner.

Modifications:

* add Kerberos support
* add OAuthBearer support
* Created a new file `KafkaConfiguration+Security.swift`
    * add `public struct` `SSLConfiguration`
    * add `public struct` `SASLMechanism`
    * add `public struct` `KerberosConfiguration`
    * add `public struct` `OAuthBearerMethod`
    * add `public struct` `SecurityProtocol`
* integrate `KafkaConfiguration.SSLOptions` in new types
* integrate `KafkaConfiguration.SASLOptions` in new types