feat(validation): introduce custom Bind rule system

[techmahedy]

Overview

This PR adds a first-class custom rule binding API (Bind::to()) to Doppar, enabling framework users and package authors to register custom validation rules directly in the pipeline without modifying core files.
Key Features

• Custom rule registration: Any class implementing RuleInterface can be registered directly in the validation pipeline.
• Conditional execution: Rules can be configured to run only under specific input conditions, ensuring flexible and context-aware validation.

Example Usage

$sanitized = $request->sanitize([
    'status' => 'required|in:active,inactive,pending',
    
    // Always runs
    'domain' => Bind::to(new Domain()),      

    // Runs only when status is 'active'
    'phone'  => Bind::to(new PhoneNumber())->context(['status' => 'active'])
]);

• If status is "active", both Domain and PhoneNumber rules run.
• If status is "inactive" or "pending", PhoneNumber is skipped silently, while Domain still runs.

How context() Works

Scenario	Behaviour
Bind::to(new Domain())	Always runs
->context(['status' => 'active'])	Runs only when $input['status'] === 'active'
->context(['a' => '1', 'b' => '2'])	Runs only when all conditions match
Condition not met	Rule is silently skipped (treated as passing)