PCKPacker: Add function to add file using PackedByteArray instead of source filename

[sinni800]

Describe the project you are working on

A game with many pck-files

Describe the problem or limitation you are having in your project

When wanting to create my pck-files, I will always have to save every resource to the file system before being able to insert it into a PCK.

This lets some things slot into place worse:

1. Unpacking a ZIP export of the project and piecemeal-adding them into PCK files
2. using EditorExportPlatform.export_project_files will call each file with a file_data PackedByteArray (it's another issue that it's hard or impossible to get a properly configurable preset with EditorExportPlatform, but that's for another day...)

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

Don't need to write many temp files

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

godot/core/io/pck_packer.cpp already gets a full byte buffer of the file which it then packs into the PCK.

It would be trivial to add another function that expects the buffer directly.

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

Only by doing this:

	var filename : String = "res://bla"
	var packer : PCKPacker # imagine it's already initialized
	var tmp = FileAccess.create_temp(FileAccess.WRITE_READ, "blabla", "bin", true)
	tmp.store_buffer(buff)
	tmp.close()
	packer.add_file(filename, tmp.get_path_absolute())
	return tmp

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

Implementing it using an external library would make it necessary to create temp files too.

Adding here, it is possible to reimplement the PCK Format (as it is rather simple) in pure GDScript (someone has actually done that in an addon), but as the core already has all the functionality, just doesn't expose this portion of it, I think it would be much less friction to spend the time to do it here.

If this proposal gets accepted, I would even do the change myself, it's really straightforward.