Add webhook templating (#23289)

Co-authored-by: Claire <claire.github-309c@sitedethib.com>

app/lib/webhooks/payload_renderer.rb

@@ -0,0 +1,67 @@
+# frozen_string_literal: true
+
+class Webhooks::PayloadRenderer
+  class DocumentTraverser
+    INT_REGEX = /[0-9]+/
+
+    def initialize(document)
+      @document = document.with_indifferent_access
+    end
+
+    def get(path)
+      value  = @document.dig(*parse_path(path))
+      string = Oj.dump(value)
+
+      # We want to make sure people can use the variable inside
+      # other strings, so it can't be wrapped in quotes.
+      if value.is_a?(String)
+        string[1...-1]
+      else
+        string
+      end
+    end
+
+    private
+
+    def parse_path(path)
+      path.split('.').filter_map do |segment|
+        if segment.match(INT_REGEX)
+          segment.to_i
+        else
+          segment.presence
+        end
+      end
+    end
+  end
+
+  class TemplateParser < Parslet::Parser
+    rule(:dot) { str('.') }
+    rule(:digit) { match('[0-9]') }
+    rule(:property_name) { match('[a-z_]').repeat(1) }
+    rule(:array_index) { digit.repeat(1) }
+    rule(:segment) { (property_name | array_index) }
+    rule(:path) { property_name >> (dot >> segment).repeat }
+    rule(:variable) { (str('}}').absent? >> path).repeat.as(:variable) }
+    rule(:expression) { str('{{') >> variable >> str('}}') }
+    rule(:text) { (str('{{').absent? >> any).repeat(1) }
+    rule(:text_with_expressions) { (text.as(:text) | expression).repeat.as(:text) }
+    root(:text_with_expressions)
+  end
+
+  EXPRESSION_REGEXP = /
+    \{\{
+      [a-z_]+
+      (\.
+        ([a-z_]+|[0-9]+)
+      )*
+    \}\}
+  /iox
+
+  def initialize(json)
+    @document = DocumentTraverser.new(Oj.load(json))
+  end
+
+  def render(template)
+    template.gsub(EXPRESSION_REGEXP) { |match| @document.get(match[2...-2]) }
+  end
+end