feat(gatsby-script): On load callback, cache

[tyhopp]

Description

Implements the ability to use an onLoad callback for scripts with sources. Usage looks like:

<Script
  src={scripts.three}
  strategy={ScriptStrategy.postHydrate}
  onLoad={(event: Event) => { console.log(`Hello world`) }}
/>

Limitations
The onLoad callback works only for scripts with sources using the post-hydrate or idle strategies. It does not work with inline scripts.

It also includes two other things:

• A cache for scripts so that loading occurs only once in all the various navigation scenarios (see test cases)
• Handling for edge case of pre-hydrate scripts loaded after client side navigation (e.g. via Gatsby link). In this case it will be treated effectively the same as post-hydrate scripts. This is a temporary fix and we should explore better solutions

Documentation

To be done in later PR(s).

Related Issues

[sc-48841]
[sc-49445]

[tyhopp]

There are a few broken e2e tests still in the gatsby script suite, will merge this first so it's not blocking and we can address later