Allow to pass functions in gatsby-config.js for browser and SSR

[mike1808]

Summary

Currently, options for the plugins in gatsby-config.js are stringified for the browser and SSR as mentioned in #13440 However, in some plugins functions are the part of the configurations. So preserving functions would be a great addition for the config file.'

Basic example

Imagine you have a plugin which calls your provided callback.

plugins: [
  { 
    resolve: 'gatsby-plugin-on-page-load',
    options: { onLoad() { sessionStorage.setItem('loaded', 'true'); }
  }
]

Motivation

In some plugins, you need to provide some kind of callback. I faced this issue with gatsby-plugin-web-font-loader where I needed to provide a callback for font load.

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[wardpeet]

This is a hard nut to crack at the moment. We have to send this function over to webpack which is a bit hard. We can stringify it but a better ways it to compile it to an AST and reconstruct it on gatsby-browser.

A workaround for this is to save this content to disk and import it inside gatsby-browser.

[joefiorini]

I running into this issue as well. I'm using a gatsby theme and I have multiple sites. Each one needs to pass an object that is used for some central configuration, and that object can have functions (it's for mocking API endpoints client-side). In the theme's gatsby-browser I take the configuration object and pass it on to a provider component. At that point it's lost the functions :(. Note that I can't import this configuration in the theme's gatsby-browser or gatsby-ssr because it is unique for each gatsby site that uses the theme. I can't seem to find anywhere else I can pass it, unless I completely change my architecture and set it up using a hook, which doesn't sound very easy either.

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