Consider adding deliveryType support

[tunetheweb]

Chrome recently added deliveryType which can be set to cached, navigational-prefetch or not empty string ''.

This is available at performance.getEntriesByType('navigation')[0].deliveryType for the document which could be useful to explain the TTFB (and hence FCP and LCP).

Should this supplement/expand the navigationType in the Metric type? Or should this be a separate field? They are not completely distinct (and may be even less so in the future if the usage of deliveryType expands), but maybe just splitting the navigate type into cached, navigational-prefetch, or navigate could be sufficient for now?

[tunetheweb]

We decided this should be a separate field rather than merged into navigationType as there are instances where you'd need both. And given it's then just a straight look up from the navigation entry (which is already available in the attribution data) there is little need to add it.

However, leaving this issue open as might add a comment similar to serverTiming comment so people are aware of this:

web-vitals/src/types/ttfb.ts

Lines 54 to 59 in dfdf57d

	/**
	* The `navigation` entry of the current page, which is useful for diagnosing
	* general page load issues. This can be used to access `serverTiming` for example:
	* navigationEntry?.serverTiming
	*/
	navigationEntry?: PerformanceNavigationTiming | NavigationTimingPolyfillEntry;