Google Analytics plugin should have configurable delay before sending Page View

[Simon-Tang]

Summary

To build on the fix for #9139 in #10917, we need to support an optional configurable delay before invoking sendPageView for sites that have custom page transitions. (An example plugin: gatsby-plugin-transition-link).

I can probably make this change in the code, but I wanted to see if anyone else had thoughts on this (e.g. the naming of the option).

Basic example

In gatsby-config.js:

{
  resolve: 'gatsby-plugin-google-analytics`,
  options: {
    pageViewDelay: 250,
  }
},

Motivation

With page transitions, such as when using the plugin linked above, the next route can be mounted after an arbitrary delay, so the current requestAnimationFrame/setTimeout in onRouteUpdate will need to be replaced with a user-configured delay in this case.

[Simon-Tang]

Fixed in #15610