Define directive fallback list for `monetization-src` CSP directive

[lukewarlow]

In CSP there's a directive fallback list defined for various CSP directives. This spec should probably define one for the monetization-src directive.

Something like:

Return << "monetization-src", "default-src" >>.

If it's not desirable to fallback to default-src I think this should be called out explicitly with a strong reasoning.