feat: add EndpointCustom augmentation interface

[jhb-dev]

Payload provides CollectionCustom, GlobalCustom, CollectionAdminCustom, GlobalAdminCustom, and FieldCustom augmentation interfaces for type-safe custom data, but Endpoint.custom was typed as an inline Record<string, any> — making it impossible to augment via TypeScript module augmentation.

Adds EndpointCustom interface in packages/payload/src/index.ts alongside the existing custom interfaces, and updates Endpoint.custom in packages/payload/src/config/types.ts to use it.

Users can now augment endpoint custom data the same way they do for collections/globals:

declare module 'payload' {
  export interface EndpointCustom {
    myProperty: string
  }
}