Allow external download for supplier orders and invoices and expose direct-download variables

[mapiolca]

Motivation

• Provide the ability to generate and expose direct-download links for supplier-side documents (supplier orders and supplier invoices) and make this feature configurable from the admin UI.

Description

• Add admin toggles SUPPLIER_ORDER_ALLOW_EXTERNAL_DOWNLOAD and SUPPLIER_INVOICE_ALLOW_EXTERNAL_DOWNLOAD in supplier_order.php and supplier_invoice.php using ajax_constantonoff to control external sharing.
• Update CommonObject to set the ECM share key when element is order_supplier or invoice_supplier and the corresponding global flags are enabled so generated documents can be shared externally.
• Add substitution variables __DIRECTDOWNLOAD_URL_SUPPLIER_ORDER__ and __DIRECTDOWNLOAD_URL_SUPPLIER_INVOICE__ in getCommonSubstitutionArray() and populate them using the object's getLastMainDocLink() when allowed.

Testing

• No automated tests were added for this change.
• Performed PHP syntax checks (php -l) on the modified files and they passed without syntax errors.
• Existing automated test suite was not modified as part of this change.

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Codex Task