revert: "refactor!: drop e-invoicing integration from erpnext (#26940)"

This reverts commit c335962827.

erpnext/patches/v12_0/add_ewaybill_validity_field.py

@@ -0,0 +1,18 @@
+from __future__ import unicode_literals
+
+import frappe
+from frappe.custom.doctype.custom_field.custom_field import create_custom_fields
+
+
+def execute():
+	company = frappe.get_all('Company', filters = {'country': 'India'})
+	if not company:
+		return
+
+	custom_fields = {
+		'Sales Invoice': [
+			dict(fieldname='eway_bill_validity', label='E-Way Bill Validity', fieldtype='Data', no_copy=1, print_hide=1,
+				depends_on='ewaybill', read_only=1, allow_on_submit=1, insert_after='ewaybill')
+		]
+	}
+	create_custom_fields(custom_fields, update=True)