EXP-5700: Change from global rollout opt-out to experiments and rollouts separately (#6888)

[gruberb]

• Adding db migration from v2 to v3 due to the flag change
• Removing the old set_global_rollout function
• Adding set_experiment and set_rollout functions
• Export four new functions via UDL
  • get_rollouts_user_participation
  • set_rollouts_user_participation
  • get_experiments_user_participation
  • set_experiments_user_participation

Marked set_global_user_participation and get_global_user_participation as deprecated

Pull Request checklist

• Breaking changes: This PR follows our breaking change policy
  • This PR follows the breaking change policy:
    • This PR has no breaking API changes, or
    • There are corresponding PRs for our consumer applications that resolve the breaking changes and have been approved
• Quality: This PR builds and tests run cleanly
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• Tests: This PR includes thorough tests or an explanation of why it does not
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  • Any breaking changes to Swift or Kotlin binding APIs are noted explicitly
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[Copilot]

Pull Request Overview

This PR transitions from a global rollout opt-out system to separate experiment and rollout participation controls, providing users more granular control over their participation in these different types of testing.

• Adds database migration from v2 to v3 to handle the flag change
• Replaces global participation API with separate experiment and rollout participation controls
• Exports four new UDL functions for getting/setting experiment and rollout participation separately
• Marks global participation functions as deprecated while maintaining backwards compatibility

Reviewed Changes

Copilot reviewed 27 out of 28 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
components/nimbus/src/stateful/persistence.rs	Implements database migration v2→v3 and adds new participation flag keys
components/nimbus/src/stateful/enrollment.rs	Adds separate functions for getting/setting experiment and rollout participation
components/nimbus/src/stateful/nimbus_client.rs	Updates client to support separate participation controls with deprecated global fallback
components/nimbus/src/enrollment.rs	Introduces Participation struct and updates evolver to use separate flags
components/nimbus/src/nimbus.udl	Exports new participation functions via UDL interface
megazords/ios-rust/Sources/MozillaRustComponentsWrapper/Nimbus/Nimbus.swift	Implements iOS wrapper for new participation controls
components/nimbus/android/src/main/java/org/mozilla/experiments/nimbus/Nimbus.kt	Implements Android wrapper for new participation controls
Test files	Updates tests to use new participation model and adds migration tests

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[gruberb]

@freshstrangemusic Can we try to get this in again?