Format-less VC validation

## Description
Provide a new validation mechanism that works on VCs of arbitrary format (e.g. plaintext JSON, JWT VC, JWT VC v2, SD-JWT)

## Motivation
Verifiers may want to support multiple credential formats at the same time but our current facilities only work with JWT-encoded credentials.

No files, tests, or entry points are named. Start by locating the existing JWT-only validation facilities and their tests, then determine how format selection is represented. Done means verifiers can validate plaintext JSON, JWT VC, JWT VC v2, and SD-JWT credentials through a common mechanism.

Written by the indexing model from the issue text.