Type-safe Security Configuration (#88)

* Type-safe Security Configurations

> Adds some of the missing properties from #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

* Fix failing test

* Changes David

Modifications:

* go from `KafkaConfiguration.Key` type into more specific types:
    * `LeafAndIntermediates` to extract the public Key
    * `PrivateKey`
    * `RootCertificate`
* `RootCertificate`: add two new options
    * `probe` (default): probe list of standard paths and make first
     certificate found root default root certificate location path
    * `disableBrokerVerification`: disable broker verification entirely
* `KafkaConfiguration+Security`:
    * turn `static func`s without parameters into `static let`s for our
      enum-like `public structs`

* Review David

Modifications:

* add `public init` to `KafkaConfiguration.SASLMechanism.KerberosConfiguration`

Author: felixschlegel