Make signer in WASM32 implementation non-owned again

[wulfraem]

The current Storage based signer (src/storage/storage_signer_owned.rs) is owned, which means that it will consume a storage given to it and trying to use the Storage afterward will run into null-pointer errors.

This has some impacts upon usage:

• null-pointer errors are very un-TypeScript-ish to say the least, and will lead to errors most probably unexpected by implementers
• avoiding these results in counterintuitive code and examples (e.g. a document has to be created before being allowed to create the identity client, as both need the storage, and the client (currently) takes ownership of it)

Storage signer should be reverted to a model where Storage is not consumed when creating the identity client in TypeScript.