[Cashtab] [extension address passing p8] Add listener function to contentscript.js and GetAddress.tsx

Summary:
T2550

Depends on D12320

After getting a request to fetch the address, respond back with the address.

Test Plan:

cd web/cashtab
npm run extension
cd ../cashtab-components
npm run storybook

Navigate to brave://extensions
Click 'update' and click 'Inspect views from background page'
Navigate to your local storybook. Click "Get Address"
In the dev console for background.js, confirm the address appears
In the dev console for your storybook, confirm the address appears
Change the active wallet in the extension
Repeat and confirm for new address

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12321