[SIWE] Differentiate content for social proof and backup message.

Summary: This differential introduces different SIWE message content for social proof and backup message.

Test Plan:

1. Ensure that siwe-utils.test.js pass during commit.
2. I executed SIWE login flow and ensured that messages that appear in rainbow app match expected content.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11757