Markdown support for the `datadog_event` operator

[NomadBlacky]

The Datadog Event API doesn't use Markdown by default.
To use Markdown, need to add text block with %%% to the event text.

The datadog_event operator will support markdown parameter.
If true, add text block with %%% to the event text.

+example:
  datadog_event>:
    title: "[TEST] digdag-plugin-datadog"
    markdown: true
    text: |
      # We can use Markdown!

      + foo
          + bar

      ```
      source
      codes
      ```