Add FXIOS-10530 Add a feature flag for the Sent from Firefox experiment

[ih-codes]

:scroll: Tickets

Jira ticket Github issue

:bulb: Description

This PR adds a feature flag to enable/disable Sent from Firefox.

:pencil: Checklist

You have to check all boxes before merging

• [x] Filled in the above information (tickets numbers and description of your work)
• [x] 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)

[mobiletest-ci-bot]

	Messages
:book:	Project coverage: 32.83%
:book:	Edited 4 files
:book:	Created 1 files

Client.app: Coverage: 30.86

File	Coverage
NimbusFlaggableFeature.swift	97.69%	✅
NimbusFeatureFlagLayer.swift	64.24%	✅

Generated by :no_entry_sign: Danger Swift against 40e65aadbb99b04ab780247a48a70377f3f45fbe