[Cashtab] Simplify extension address sharing

Summary:
Cashtab sends the shared address in the approval message so the extension does not need to fetch it after the approval.

Note: will not publish this until next minor version, D18647

Test Plan:
npm run extension

cd modules/cashtab-connect/demo
npm ci
npm start

address sharing works

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18664