Replace `@Polymorphic` with `JsonElement` for `credentialSubject` in `VerifiableCredential`

[etnafed]

Hello,

The current implementation of at.asitplus.wallet.lib.data.VerifiableCredential uses @Polymorphic annotation on the credentialSubject property:

@Polymorphic
@SerialName("credentialSubject")
val credentialSubject: CredentialSubject

This approach assumes that the serialization mechanism can discriminate between different CredentialSubject implementations, typically using a type field as a class discriminator. However, according to the W3C Verifiable Credentials Data Model 1.1 specification, there is no guarantee that a type field will be present in the credentialSubject object.

Problem

The polymorphic serialization strategy currently employed may fail or behave unexpectedly when:

• The credentialSubject does not contain a type field
• The credentialSubject structure varies significantly between different credential types
• Interoperability with credentials from external issuers that don't follow the assumed discriminator pattern

This creates a potential incompatibility with valid W3C Verifiable Credentials that don't include the expected discriminator field.

Proposed Solution

Replace the @Polymorphic annotation and CredentialSubject type with JsonElement to handle the credentialSubject as a flexible JSON structure:

@SerialName("credentialSubject")
val credentialSubject: JsonElement

Benefits

• ✅ Spec compliance: Aligns with W3C VC Data Model 1.1 specification
• ✅ Flexibility: Handles any valid JSON structure for credentialSubject
• ✅ Interoperability: Works with credentials from any compliant issuer
• ✅ No assumptions: Doesn't require a specific discriminator field

Impact

This change would affect:

• Any code that currently expects credentialSubject to be a specific CredentialSubject subtype
• Serialization/deserialization logic that relies on polymorphic behavior
• Type-safe access to credential subject properties (would require manual JSON parsing)

I've tested it locally, the openid4vci and openid4vp flows still works after those modifications. I can propose a pull request to fix this.

Best regards

[JesusMcCloud]

@n0900

[etnafed]

I've created a pull request : #502