fix(sandbox): allow the sandbox to load plugins when running locally

[demetriusfeijoo]

What?

Add the allowAllOrigins option to the createFieldPlugin function.

Why?

JIRA: EXT-2258

It allows our demo package (or any other plugin created by other developers) to be loaded in a local running instance of the Sandbox.

How to reproduce?

From the main branch run:

yarn
yarn dev:demo
yarn dev:container

Then, this error should appear in your console:

and the demo plugin should not be loaded:

How it should work after the change?

Now, from the task's branch, run these commands:

yarn
yarn dev:demo
yarn dev:sandbox

Then, no error should appear in your console:

and the demo plugin should be loaded:

PS: The Vercel action is not running yet since it wasn't modified to consider the new path (packages/sandbox) and also the new project name (modified here) in its settings.