Refactor public interfaces

[elf-pavlik]

Given all the functionality in place and experience with implementing both authorizations agent and demo applications, we can make quite a lot of improvement.

Suggested in #51 (comment)

@angelaraya: Also, while I like compressing parameters into option objects in this case it obfuscates the inputs needed by the function. I'd rather have as many parameters as needed here, specially because the type name ShareDataInstanceStructure does not convey at all what it actually is.

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• Strategy for handling invalid data #52
• Reconsider what DataInstance should extend #28