[landing] Introduce SIWENonceContext

Summary: This will allow us to get the siweNonce from landing-handler to the SIWE component without a bunch of prop drilling (which is what I did initially to try to keep things simple but it was a lot).

Test Plan: Will be tested implicitly by subsequent diffs.

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D5975