Add sign message functionality

[H34D]

Story

As a USER who is using a dApp, I want to sign a message instead of a transaction so I can use formal verification of a signature in the dApp to transact in the behalf of the User like Opensea on Ethereum.

Acceptance Criteria

• [x] Message to sign should be a string, this can either be an wallet address or a serialised json object
• [ ] The signature and public key should be returned in Base58 encoding so other NEAR Components can pick it up
• [x] The message to sign should be displayed nicely
• [x] The signing flow should be analog to the flow when signing transactions
• [x] Tested

Unhappy Path

The user cancels the signing operation

Assets

• Mockup:

NOTES

PR Open at: #2260