Http post2 alternate jinja root

[akusei]

Description

• Added support for multiline JSON strings in YAML fields (similar to alert_text)

    http_post2_payload: |
      {
        "description": "An event came from IP {{ _data["client.ip"] }}",
        "username": "{{ _data['username'] }}"
        {%- for k, v in some_field.items() -%}
        ,"{{ k }}": "changed_{{ v }}"
        {%- endfor -%}
      }
    http_post2_headers: |
      {
        "authorization": "Basic 123dr3234",
        "X-custom-{{key}}": "{{type}}"
      }

• Added jinja_root_name support to the HTTP POST2 alerter
• Changed the --data argument for rule testing to accept both a file with a single JSON document or a file with a proper JSON list of objects
  Example file with single JSON object

{"fieldname": "value"}

Example file with list of JSON objects:

[{"fieldname":"value1"},{"fieldname":"value2"}]

• Additional error checking has been added around JSON decoding and jinja templates for HTTP POST2 headers and payloads

Checklist

• I have reviewed the contributing guidelines.
• I have included unit tests for my changes or additions.
• I have successfully run make test-docker with my changes.
• I have manually tested all relevant modes of the change in this PR.
• I have updated the documentation.
• I have updated the changelog.

Questions or Comments

The change to --data for rule testing was added just for convenience. I was doing some testing and didn't realize the document needed to be a list of JSON objects. I didn't see anything stating it needed to be a list so I changed the code to allow for a single JSON object in the file or a list of JSON objects. The intent of this PR was not to make this change so if it's not wanted I'm okay with removing it

[jertel]

Nice job on this PR and the extensive unit test coverage. Thank you!