Configure global defaults and update during runtime

[pascalbaljet]

This PR brings support for settings configuration defaults, and the ability to update them during runtime. Currently, it supports three configuration settings, and this might be expanded in the future.

You can pass a default config object to the createInertiaApp() method. You don't have to pass all keys, just the ones you want to tweak.

createInertiaApp({
  // resolve, setup, etc.
  defaults: {
    form: {
      recentlySuccessfulDuration: 5_000,
    },
    prefetch: {
      cacheFor: 60_000,
    },
    visitOptions: (href: string, options: VisitOptions) => {
      return { headers: { ...options.headers, 'X-Foo': 'Bar' } }
    },
  },
})

The most interesting one here is the visitOptions callback, which acts more or less as an interceptor for Inertia requests, giving you the ability to change the options before the request is sent.

Each adapter now also exports a config instance to update the values during runtime. It supports the dot-notation.

import { config } from '@inertiajs/vue3'

config.set('form.recentlySuccessfulDuration', 1000)

You may also update multiple values at once by passing an object.

config.set({
  'form.recentlySuccessfulDuration': 1000,
  'prefetch.cacheFor': '5m',
})

Fixes #1669.

[danmatthews]

@pascalbaljet is there a way to make visitOptions here apply to router.visit() calls as well?