code: def add_key(xpub_or_key, signer_conn)
        xpub = xpub_or_key.is_a?(MockHSM::Key) ? xpub_or_key.xpub : xpub_or_key
        @xpubs_by_signer[signer_conn] ||= []
        @xpubs_by_signer[signer_conn] << xpub
        @xpubs_by_signer[signer_conn].uniq!
      end
docs: Add a new key/signer pair to the HSM signer.
   @param [MockHsm::Key || String] xpub_or_key An object with an xpub key, or an xpub as a string.
   @param [Connection] signer_conn Authenticated connection to a specific HSM instance.