Add webpage interaction to extension

Summary: CashTab as an extension can be used to complete a transaction defined on a webpage, similar to Badger Button

Test Plan:

1. Follow steps in web/cashtab/README.md to build and test extension wallet
2. Navigate to https://laughing-villani-8cfcaf.netlify.app/?path=/story/badgerbutton--default and click the green button
3. Observe that the CashTab extension opens with the Send form populated from the web page
4. Click send. Observe that an additional confirmation modal is required to complete the transaction on a web-loaded tx.

npm run extension

Reviewers: alcipir, majcosta, #bitcoin_abc

Reviewed By: majcosta, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D8734