chore: add scheduled_flag_update table and proto message

[cre8ivejp]

Part of #893

Things done

• Added scheduled_flag_update table
• Added scheduled_flag_update proto message

Queries examples

Query 1: Filter by Change Type and Field Type Using JSON_CONTAINS

SELECT *
FROM scheduled_flag_update
WHERE JSON_CONTAINS(
  changes,
  JSON_OBJECT('change_type', 'SCHEDULED_CHANGE_CREATE', 'field_type', 'FIELD_TYPE_VARIATIONS')
);

Query 2: Filter by Rule ID and Variation ID Using JSON_CONTAINS

SELECT *
FROM scheduled_flag_update
WHERE JSON_CONTAINS(
  changes,
  JSON_OBJECT(
    'field_type', 'FIELD_TYPE_RULES',
    'field_value', '{"id": "rule-123", "strategy": {"fixed_strategy": {"variation": "variation-456"}}}'
  )
);

---

This pull request includes significant changes to the database schema, protobuf definitions, and related code. The main updates involve the creation of a new table for scheduled flag updates, the removal of old protobuf message types, and the addition of new protobuf definitions for scheduled updates.

Database Schema Changes:

• Created scheduled_flag_update table with necessary fields and foreign key constraints in migration/mysql/20250226091538_create_scheduled_flag_update_table.sql.
• Updated migration/mysql/atlas.sum to include the new migration file for the scheduled_flag_update table. [1] [2]

Protobuf Changes:

• Removed ScheduleUpdate and ScheduleUpdateListValue message types from proto/feature/feature.pb.go and proto/feature/feature.proto. [1] [2] [3] [4] [5]
• Added new protobuf definitions for ScheduledChange and ScheduledFlagUpdate in proto/feature/scheduled_update.proto.

UI Changes:

• Updated ui/web-v2/src/proto/feature/scheduled_update_pb_service.js to reflect the new protobuf definitions.

[cre8ivejp]

The field types correspond to the fields found in the Feature proto message.
We can add new field types in the future if needed.

[cre8ivejp]

The allowed change types.

[Ubisoft-potato]

Nice design!

[hvn2k1]

The design LGTM!