Equivalent for removed `legacyKeyLookup`

Hi, as pointed out in this PR/comment we are looking for some equivalent of the [dropped](https://github.com/torusresearch/torus.js/commit/579f9d1e53d637df66cd08188e82f4867f62bca8) `legacyKeyLookup` method, i.e. a method that allows to check for the presence of a key in a verifierId without the call resulting in a creation of the key if it doesn't exist (which is currently the side effect of `GetPubKeyOrKeyAssign` call). Is there any such method, and if not, could you please introduce it?

Our usecase is detection of whether a user has ever created a key on a verifier or not and based on that we have some custom migration logic

Related comment/PR: https://github.com/torusresearch/torus.js/pull/132#issuecomment-2222242439

Start by reading the dropped legacyKeyLookup implementation and the related PR 132 comment, then trace the current GetPubKeyOrKeyAssign behavior. Done means providing an equivalent presence check that does not create a key when none exists, so callers can detect prior verifier key creation for migration logic.

Written by the indexing model from the issue text.