ExtrinsicSigner struct does not allow clone

[haerdib]

I've run into the problem that the ExtrinsicSigner created in the worker does not seem to be cloneable. I haven't looked further into this yet. Therefore, I see the following TODOs for this issue:

Steps:

• Try to reproduce by creating an unit test and clone the ExtrinsicSigner
• Fix if possible & reproducible -> not possible because the Unlookup struct from substrate does not implement clone.

Workaround: Implementing a new ExtrinsicSigner that has a hardcoded Address type: see #511

But before investing too much time, this issue may be obsolete after finishing #406.