[Remote Tab Management] Receive Close Tab commands from other devices

[skhamis]

:scroll: Tickets

Jira ticket Github issue

First of the Remote Tab Management PRs. Couple of things here:

• Add Nimbus feature flag to be able to control it while developing on it
• Added a secret menu option
• Renamed SendTabDelegate to FxACommandsDelegate to encompass CloseTab command and future ones
• Upon receiving a "CloseTab" command from FxA, go through all tabs and close the first matching Url

:pencil: Checklist

You have to check all boxes before merging

• [ ] Filled in the above information (tickets numbers and description of your work)
• [ ] Updated the PR name to follow our PR naming guidelines
• [ ] Wrote unit tests and/or ensured the tests suite is passing
• [ ] When working on UI, I checked and implemented accessibility (minimum Dynamic Text and VoiceOver)
• [ ] If needed, I updated documentation / comments for complex code and public methods
• [ ] If needed, added a backport comment (example @Mergifyio backport release/v120)