Add methods to add, remove and manage files to be added to the PCK in EditorExportPreset

[citizenll]

Describe the project you are working on

I am working on a project using the Godot engine that involves exporting game assets as .pck files. This project requires a flexible export process that can accommodate different configurations based on specific project needs.

Describe the problem or limitation you are having in your project

The current limitations of the ExportPrset interface hinder the ability to create custom export plugins effectively. Specifically, it lacks the necessary internal interfaces to handle .pck file processing with the required flexibility, making it difficult to export different sets of files based on dynamic configurations.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

The proposed enhancement involves exposing additional internal interfaces within the ExportPrset. This enhancement will allow developers to access export data more effectively and provide the capability to selectively export .pck files based on a user-defined file list during the export process.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

I have submitted the PR. #102046

If this enhancement will not be used often, can it be worked around with a few lines of script?

cannot

Is there a reason why this should be core and not an add-on in the asset library?

Need to export internal interfaces; plugins cannot achieve this.