[native] Generate primaryIdentityPublicKey and pass to landing/SIWE

Summary:
Make necessary JSI calls to get primaryIdentityPublicKey and set siwe-primary-identity-public-key header before loading landing/SIWE webview.

Depends on D6115

Test Plan:
Logged primaryIdentityPublicKey value and ensured it was as expected.

In subsequent diff will capture the value from header and pass to landing/SIWE component so it can be included in the signed SIWEMessage.

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6124