fix: type augmentation of RequestContext

[r1tsuu]

What?

Makes this to actually work

import type { RequestContext as OriginalRequestContext } from 'payload'

declare module 'payload' {
  // Create a new interface that merges your additional fields with the original one
  export interface RequestContext extends OriginalRequestContext {
    myObject?: string
    // ...
  }
}

Why?

This is described in our docs https://payloadcms.com/docs/beta/hooks/context#typescript therefore it should work.

How?

In order to get the declaration work, we need to reuse the type from the root file payload/src/index.js. Additionally, removes RequestContext type duplication in both payload/src/types/index.js and payload/src/index.js.

Fixes #8851

[github-actions]

🚀 This is included in version v3.0.0-beta.125