feat: add support for nested list handling in webhook request content mapping

[Qoyyuum]

This PR fixes #14

[seagill-huntress]

This comment is directed at any maintainer looking to merge this.

This section of original code still causes a bug where simple string values in dicts in a list such as Slack Blocks formatting get dropped.

For example:

"request_body": {
               "text": "[${{alerts.severity.severity_name}}] ${{alerts.alert_title}}",
                "blocks": [
                    {
                        "type": "header",
                        "text": {
                            "type": "plain_text",
                            "text": "[${{alerts.severity.severity_name}}] ${{alerts.alert_title}}"
                        }
                    },
...

Leads to parsed value of

{"text" : "[Low] Test Alert", "blocks": [{"type": "", "text": {"type": "", "text": "[Low] Test Alert"}}] ...}

Removing this entire section fixes the issue and I'm not even clear why this section is here as it seems unnecessary given that the expectation is the config should be valid JSON and should not be trying to interpret "some.key" as a path rather than a simple JSON key name.

Otherwise, this patch seems to work correctly and solved my problem with this module.