fix: PHP 7.4 typed property compatibility with Event Tickets 5.6+

[GREENFONTS]

Summary

This PR fixes fatal activation errors when using the Paystack plugin with Event Tickets 5.6.0+, which introduced PHP 7.4+ typed properties.
Without these updates, the plugin fails to activate due to type mismatches and missing interface implementations.

Reported issue: https://wordpress.org/support/topic/paystack-plugin-not-compatible-with-event-ticket/

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Changes

Gateway class

• Added string type declarations to $key, $settings, and $merchant
• Added array type declaration to $supported_currencies
• Added helper method is_currency_supported($currency_code)

Order_Endpoint class

• Added string type declaration to $path

Merchant class

• Implemented required interface methods:
  • get_client_secret()
  • get_disconnect_action()
  • get_disconnect_url()
• Updated save() method to properly handle needs_save() and log results

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Fixes

• Resolves fatal errors caused by PHP 7.4 typed properties:
  • Type of Gateway::$key must be string
  • Type of Order_Endpoint::$path must be string
  • Class Merchant contains abstract methods and must implement Merchant_Interface

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Testing

• Tested with Event Tickets 5.6.0+
• Tested on PHP 7.4+
• Plugin now activates successfully and functions as expected