[Spec] Refactor fenced frames spec to decouple core and consumer APIs functionality

[shivanigithub]

Keeping the core fenced frames functionality in the spec and moving the consumer API specific parts to the respective APIs that can then link into the core fenced frames spec. For example, fields in FencedFrameConfig that will be removed from the core FF spec to the respective API specs:

• https://wicg.github.io/fenced-frame/#fencedframetype-exfiltration-budget-metadata,
• https://wicg.github.io/fenced-frame/#fenced-frame-config-fenced-frame-reporting-metadata

/cc @blu25 @VergeA