Add support for sapphire-paratime 2.x

[matevz]

This PR:

• wraps the provider with the new sapphire-paratime 2.x API
• deprecates the signed query approach to fetch the message
• uses SiweAuth to authenticate and retrieve the message
• removes obsolete workaround for eth_requestAccounts (fixed in frontend: Use eth_requestAccounts instead of eth_accounts #17)

Merge after oasisprotocol/sapphire-paratime#332 and oasisprotocol/sapphire-paratime#303

TODO: Wait for sapphire-contracts to be deployed on npmjs (oasisprotocol/sapphire-paratime#490). Then update the lock file.

[rube-de]

This PR should be now merged in the vue branch.

[lubej]

This PR should be now merged in the vue branch.

This would break, current React rewrite, as it is based on "v1". Can we hold off merging? Until rewrite of frontend is done on top of this PR?

cc: @matevz

[lubej]

@matevz I didn't want to mess with your branch, thats why I created #24. Could you double check my PR? And I can rebase this one on top of mine, or if you want to do that, that is fine as well.

[lubej]

LGTM.