Improve KMS service configuration

[robobario]

          Can we make this an actual issue please?

Originally posted by @SamBarker in #7 (comment)

In this branch I wanted to create a MockKms with a map from key reference to key. We are limited to a couple of Strings at the moment on the KmsDefinition:

    private String instanceId; // optional. IBM Cloud requires this
    private String credential;

So to configure in the map, I abused the credential field to pass in a CSV to build the map.

Maybe topic-encryption could pinch the Kroxylicious Contributor API so each KMS type could optionally define it's own custom configuration class that would be constructed and wired into the constructor.

Or it could support a Map<String, Object> of generic configuration.