[landing] Add siwePrimaryIdentityPublicKey to SIWEContext

Summary:
Add siwePrimaryIdentityPublicKey to SIWEContext so the value is available throughout landing without prop drilling. Specifically we'll be using siwePrimaryIdentityPublicKey in siwe.react.js where we'll need the public key so it can be included in the SIWE message that gets signed by the user's wallet.

Depends on D6127

Test Plan:
Able to retrieve the value of siwePrimaryIdentityPublicKey in siwe.react.js:
https://blob.sh/ef0479.png (yeah contrast isn't great, but the public key is there as expected).

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6128