Update script to handle beta testing users and update opted in logic

[erikh360]

This pull request introduces support for importing beta testing users as opted out during the migration process and adds a migration key to identify the batch of migrated users. The changes ensure that the opt-in status can be overridden for specific migrations and that tests cover this new behavior.

Migration logic enhancements:

• Added IMPORT_AS_OPTED_OUT and MIGRATION_KEY flags in fetch_rapidpro_contacts.py to allow importing users as opted out and tagging them with a migration batch key.
• Modified the process_opted_in function to accept an override parameter, ensuring users are imported as opted out when required.
• Updated the contact field processing logic to use the override flag and include the migration key in user data. [1] [2]

Testing improvements:

• Added and updated tests in test_fetch_rapidpro_contacts.py to verify that the opt-in override and migration key are correctly applied and exported. [1] [2] [3] [4] [5]
• Added a test in test_process_fields.py to confirm that the override always sets opted_in to false, regardless of input.