Implement `add_signature_or_message` method on ProposedBundle

Implement a method for adding signatures or messages in bundle transactions starting at an offset.

Reference implementation is found [here](https://github.com/iotaledger/iota.js/blob/c2908c33cc4fb7acd38b30df2dfd282520e1a05d/packages/bundle/src/index.ts#L242). 
