LG-13665: Add option to change selected email shared with partner

[aduth]

🎫 Ticket

LG-13665

🛠 Summary of changes

Adds a new flow to allow a user to change their selected email shared with a partner from their Connected Accounts dashboard screen.

📜 Testing Plan

1. Go http://localhost:3000
2. Sign in
3. From account dashboard, click "Connected Accounts" in sidebar
4. (If you don't have any connected accounts, sign in and consent using sample application)
5. Observe:
   1. If you have selected a specific email to be changed (either from consent screen or from the following steps), it is shown
   2. Otherwise, you see "Email not yet selected"
6. Click "Change"
7. Change your selected email (you may need to add a second email first, unless you have "Not yet selected")
8. Click "Submit"
9. Repeat Step 5

👀 Screenshots

[aduth]

Will edit this ticket number once available, before merging. We're implementing this as English-only for now, since it's behind a feature flag. The referenced ticket will implement translations once they're available.

Related Slack discussion: https://gsa-tts.slack.com/archives/C05R6BLVAQG/p1725033025669399

[aduth]

To do: Write spec for this view. Ideally include spec covering be_logically_grouped expectations, avoiding similar regression addressed for email language in 7f4c413 & 4b256c1 .