Fix HTTP post 2 rule for nested keys + safeguard for non-string headers

[lepouletsuisse]

Description

• Fix an issue where using nested keys in the payload break the HTTP Post 2 alerter with a TypeError: Can't compile non template nodes error.
• Make the Jinja2 templating simpler by stringifying the dict to replace the values, then change it again to a dictionary for the request.
• Add a safeguard to avoid being able to send nested headers in the request (Headers are always strings)

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

I'm the one that created this Alerter, but I forgot to handle nested keys in the payload.

[jertel]

Thanks for the alerter improvements!