[keyserver] Capture siwe-nonce header and make available to SIWE component

Summary:
Capture the siwe-nonce header (which will be passed via the WebView component on native) from the req object and make available to SIWE component in landing via SIWENonceContext.Provider.

Depends on D5975

Test Plan:
Able to retrieve value in SIWE as expected:

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D5977