Library should have better interfaces for properties that are required and always present in SD-JWT vcs

[TimoGlastra]

E.g. payload for SD-JWT VC MUST include vct and some other properties. So that means:

• the input payload must have the property
• and after validation (that checks if vct is present) the property should also be there

currently almost all interfaces are types as Record<string, unknown> for the header and payload, which makes the library API less nice to work with