Fix PHP typed properties to match parent classes

[gugaalves]

This pull request addresses compatibility issues with newer versions of the Event Tickets plugin (5.6.0+) that use PHP 7.4+ typed properties. Without these changes, the plugin produces fatal errors when activated alongside recent versions of Event Tickets (as shared at https://wordpress.org/support/topic/paystack-plugin-not-compatible-with-event-ticket/).

Changes Made

1. Gateway Class:

   • Added string type declaration to $key property
   • Added string type declaration to $settings property
   • Added string type declaration to $merchant property
   • Added array type declaration to $supported_currencies property

2. Order_Endpoint Class:

   • Added string type declaration to $path property

3. Merchant Class:

   • Implemented missing interface methods required by Merchant_Interface:
     • get_client_secret(): Returns null as Paystack doesn't use client secrets in the same way as other payment gateways
     • get_disconnect_action(): Returns the action name for disconnecting the merchant
     • get_disconnect_url(): Returns the URL for disconnecting the merchant

Fixes

These changes resolve the following fatal errors:

• Fatal error: Type of paystack\tec\classes\Gateway::$key must be string (as in class TEC\Tickets\Commerce\Gateways\Contracts\Abstract_Gateway)
• Fatal error: Type of paystack\tec\classes\REST\Order_Endpoint::$path must be string (as in class TEC\Tickets\Commerce\Gateways\Contracts\Abstract_REST_Endpoint)
• Fatal error: Class paystack\tec\classes\Merchant contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (TEC\Tickets\Commerce\Gateways\Contracts\Merchant_Interface::get_client_secret)

Testing

These changes have been tested with Event Tickets plugin version 5.6.0+ and PHP 7.4+. The plugin now activates successfully and functions as expected.

Background

The Event Tickets plugin has been updated to use PHP 7.4+ typed properties, but the Paystack for Events Calendar plugin hasn't been updated to match these changes. This PR ensures compatibility between the two plugins.