feat: update metadata fields from the challenger content

[shsteimer]

Description

Related Issue

Fix #23

Motivation and Context

How Has This Been Tested?

Tested with a customer on their live website where they were relying on the template meta property for how things rendered.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[shsteimer]

I considered defaulting this to ['template', 'theme'] but reverted to empty to avoid this being a breaking change. If we aren't concerned about a breaking change, I think the list above is reasonably safe and should cover most cases where metadata affects rendering of the page