Add flag to show whether intrasite navigation

If the page is an intra-site navigation, then it likely has different performance characteristics to a cross site navigation which may have a higher TTFB and/or less objects cached.

This could be measured with `new URL(document.referrer).origin === new URL(performance.getEntriesByType("navigation")[0].name;)`

Where should this be exposed? In the TTFB attribution model?
Or as general additions to the [Metric](https://github.com/GoogleChrome/web-vitals#metric) object so available for all metrics?

Start with the Metric object documentation and the TTFB attribution model named in the issue, then review the proposed document.referrer and navigation performance-entry check. Resolve whether the flag belongs in TTFB attribution or all Metric objects; done requires a decided exposure and defined behavior.

Written by the indexing model from the issue text.