Add MongoDB schema validation and migrations

[anitnilay20]

Description

Implement schema validation and migration system for MongoDB collections.

Requirements

• JSON Schema validation
• Collection schema management
• Data migration utilities
• Schema versioning
• Validation rule enforcement
• Write comprehensive tests

Acceptance Criteria

• JSON Schema integration
• Collection validation rules
• Migration system for data changes
• Schema evolution support

Files to create

• beatrix_core/src/mongo/schema.rs
• beatrix_core/src/mongo/migrations.rs

Example Usage

#[derive(MongoModel)]
#[schema(validate = "user_schema.json")]
struct User {
    #[validate(regex = "^[a-zA-Z ]+$")]
    name: String,
    #[validate(format = "email")]
    email: String,
}

Migration::new("add_user_status_field")
    .collection("users")
    .add_field("status", "active")
    .run(&db).await?;