Make API more ergonomic

[redlicha]

• use distinct and descriptive types instead of generic string and int where appropriate, e.g. for the various IDs (cloudspace ID vs. account ID vs. machine ID ...)
• make it clear which parameters are required and which ones are optional in the API (parameter struct with constructor with functional options? / setters for optionals?)
• use enums/constants for things like disk type

To maintain backward compatibility, this could be offered on top of the existing API

Followup to #38 (comment)