Is it possible to sign verifiable credential with a did:web ?

## Description

I'm currently evaluating identity.rs for an SSI context.
Everything looks good except it seems we can only sign a VC with an `Account` structure who can only manage an iota DID.

Is it correct ?

## Motivation

Evaluate `identity.rs` in an interoperable context (with many DID resolver).

## Requirements

Just a question right now.

## Are you planning to do it yourself in a pull request?

Depending on the response.

The issue mentions identity.rs, Account, iota DID, did:web, and verifiable credentials, but no file or test. Start by tracing how Account is used for VC signing and how DID methods are resolved; done would require a documented answer or a defined implementation path for did:web support.

Written by the indexing model from the issue text.