implemented method which get invoice number from billins side and updated tests

app/services/eis_billing/get_invoice_number.rb

@@ -0,0 +1,27 @@
+module EisBilling
+  class GetInvoiceNumber < EisBilling::Base
+    attr_reader :invoice
+
+    def self.send_invoice
+      base_request
+    end
+
+    private
+
+    def self.base_request
+      uri = URI(invoice_generator_url)
+      http = Net::HTTP.new(uri.host, uri.port)
+      headers = {
+        'Authorization' => 'Bearer foobar',
+        'Content-Type' => 'application/json',
+        'Accept' => TOKEN
+      }
+
+      http.post(invoice_generator_url, nil, headers)
+    end
+
+    def self.invoice_generator_url
+      "#{BASE_URL}/api/v1/invoice_generator/invoice_number_generator"
+    end
+  end
+end