Credential validation process takes public key as parameter instead of attempting to fetch it

## Description
When re-design credential (plaintext, JWT, SD-JWT, SD-JWT VC, and so on) validation don't attempt to fetch the verification key by analyzing the credential itself, instead have the user supply it.

## Motivation
It is impossible to take into account every possible way an issuer might reference the verification key in the credential itself.

No files or tests are named; begin by locating the credential validation entry points for plaintext, JWT, SD-JWT, and SD-JWT VC credentials. Define the affected API boundary and verify that validation uses a caller-supplied public key rather than fetching one from the credential, consistently across those formats.

Written by the indexing model from the issue text.