[4.x] Allow custom Echo client configuration

[jacksleight]

This PR allows you to use a custom broadcasting Echo client config within the CP.

It adds a new Statamic.$echo.config() method and passes some additional pusher options through to the JS side.

For example to use with Soketi add this to cp.js:

Statamic.booting(() => {
    
    Statamic.$echo.config(() => ({
        broadcaster: "pusher",
        key: Statamic.$config.get('broadcasting.pusher.key'),
        cluster: Statamic.$config.get('broadcasting.pusher.cluster'),
        wsHost: Statamic.$config.get('broadcasting.pusher.host'),
        wsPort: Statamic.$config.get('broadcasting.pusher.port'),
        wssPort: Statamic.$config.get('broadcasting.pusher.port'),
        forceTLS: false,
        encrypted: true,
        disableStats: true,
        enabledTransports: ["ws", "wss"],
    }));

});

[jasonvarga]

I made some tweaks:

• The config gets passed into the callback instead of the Echo object.
• Multiple config callbacks can be used. Maybe multiple addons would want to modify the config.

We should document that you should modify the config rather than defining a completely new one.

Statamic.$echo.config(config => Object.assign(config, {
    some: 'new variable',
    key: config.key + ' but modified',
}));

Although if you know you don't have any addons etc that would also be modifying it, you can still go ahead and just override the whole thing.

Statamic.$echo.config(() => ({
  your: 'new',
  config: 'here',
})