Add FF_OPT_IN_EMAIL_ENABLED Feature Flag

[AleDore]

In order to enable opt in email we need a Feature flag that could drive our beta phase without impacting default behaviour before the release date.

List of Changes

• Add FF_OPT_IN_EMAIL_ENABLED env variable
• Update StoreMessageContentActivity
• Add Unit test based on new env variable

Motivation and Context

This PR allows us to enable opt in email at the right moment

How Has This Been Tested?

Unit test

Screenshots (if appropriate):

Types of changes

• Chore (nothing changes by a user perspective)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.

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[BurnedMarshal]

Can we merge this? Is needed a rebase first.