[native] added webview to login with working SIWE (registration only)

Summary:
linear issue
THIS IS A PoC
added a third button to login with a SIWE prompt that opens landing, signs a message, and registers with username: address, password: signature
obviously that is not the final form of registration/login

originally, i had added another react navigation screen to the account modals. from a UX perspective, i like the idea of matching the other sign in experiences instead of presenting a full modal screen. it's definitely ugly tho.
i think the endgame is a bottomsheet modal that pulls up to about ~half the screen, that way we can re-use it anytime we need to sign messages.

Test Plan: this only works in the sim as of right now. run /landing and make sure you have a wallet on a physical device. when rainbowkit prompts you to connect, tap walletconnect & then "scan QR" in the top right. depends on D4997

Reviewers: atul, ashoat, abosh, varun!

Reviewed By: ashoat

Subscribers: varun, tomek, jacek, abosh, ashoat, atul

Differential Revision: https://phab.comm.dev/D5023