Finetune builder implementations, deserialize with function to uphold certain OBv3 rules

[Oran-Dan]

Description

The large json schema reuses structs in several places, often with an intermediary enum: SingleItem,Vec. These enums do not yet have full implementations to keep the building pattern smooth.

Other than that some fields have mandatory entries. Mainly type with the mandatory entries: "VerifiableCredential" and "OpenBadgeCredential". We need to implement a deserialize with function that checks this.

Some structs also need a deserialize with function to check that at least one of the fields, which are all optional in these specific structs, are not empty, as the specs demand.

Motivation

Finetuning this OBv3 for rust library

Resources

https://www.imsglobal.org/spec/ob/v3p0

To-do List

• Item one
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