SignMessage feature

[pawelstepien-da]

Handle CIP-0103 singMessage method in Wallet Gateway.
It should sign an arbitrary string using primary wallet's signing provider and return the signature.
Most likely should render separate view from current approve transaction, eventually approve view could be more generic to accommodate both cases.

[mjuchli-da]

As part of #1692 I've implemented the asynchronous signMessage flow as shown in the figure above.

[mjuchli-da]

Closed via #1718