Use JSON Schemas to validate type property existence requirements

[KendallWeihe]

Created as a result of the work of #109

We have standardized DID and VC type requirements as defined here and here. And part of that standardization is a requirement for the existence of some type properties. For example, a VC issuanceDate is always required to exist, but a VC expirationDate is not required to exist.

Let's use the JSON Schemas to enforce this requirement.

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Design proposal

Two possible approaches, and we could choose to do both

1. We could add functions specific to validating, which the consuming app is expected to call -- this would be useful given they already have the type instantiated and didn't use the json marshalling functionality
2. We could write custom json unmarshalling/marshalling functions (ex how we're doing this in jwt/jwt.go here) -- this would be useful if they were unmarshalling/marshalling the data

[KendallWeihe]

Some good inspiration on how this could be implemented from over in tbdex-go

• https://github.com/TBD54566975/tbdex-go/blob/main/tbdex/validator.go
• and then custom unmarshal to validate during unmarshaling https://github.com/TBD54566975/tbdex-go/blob/9ef8c2f858e55ede5795425ec8b0a37e5e1d5c6e/tbdex/offering.go#L112