Upgrade SDK to support Payload API V2

[ianhalpern]

Description

This PR adds support for API v2 by implementing the X-API-Version header functionality and adding new API v2 object classes to the PHP SDK.

This update allows developers to specify which version of the Payload API they want to use by setting Payload\API::$api_version. When set, all API requests will include an X-API-Version header with the specified value, enabling compatibility with API v2 endpoints and features.

Changes include:

• Add API::$api_version static property for API version configuration
• Add X-API-Version header to all API requests when API::$api_version is set
• Add 12 new API v2 object classes: Profile, BillingItem, Intent, InvoiceItem, PaymentAllocation, Entity, Stakeholder,
  ProcessingAgreement, Transfer, TransactionOperation, CheckFront, and CheckBack
• Update import.php to include all new object classes
• Create comprehensive unit tests with mocked curl calls for X-API-Version header functionality

Type of change

• New feature (non-breaking change which adds functionality)
• Enhancement (non-breaking change which enhances functionality)

Usage Example

<?php
require_once('import.php');

// Set API credentials
Payload\API::$api_key = 'your_api_key';

// Enable API v2
Payload\API::$api_version = 'v2.0';

// All requests will now include X-API-Version: v2.0 header
$profile = Payload\Profile::create([
    'name' => 'Example Profile'
]);

[RickRen7575]

Want to add an example using sessions?

[ianhalpern]

PHP doesn't have session support yet.