Implement EditorTextureImportPlugin to extend texture importer

[beicause]

Partially implements godotengine/godot-proposals#1943
This PR intends to address the situation that many proposals need options to customize texture importer:

• Allow remapping the color channels of a texture on import godot-proposals#11243
• Add options for resize and trim alpha processing in texture importer godot-proposals#11997
• Allow to invert texture colors in the default importer godot-proposals#9618
• Add "Alpha from GrayScale" texture import setting godot-proposals#5247

Related PRs:

• Option for Texture Importer to ignore image alpha #81803
• Invert color channels setting in Texture2D importer #91284
• Add Channel Remap settings to ResourceImporterTexture #99676
• Add Alpha from Grayscale import option to Texture2D #88669
• Add an import option to correct a heightmap texture's baseline or normalize it #100226
• Implement an importer to support Lottie animation #91580

EditorTextureImportPlugin can be used to extend ResourceImporterTexture and modify the process of importing CompressedTexture2D, allowing to load custom file format, add importer options, and change the image when processing.

The design mainly refers to the EditorScenePostImportPlugin.

Example:

@tool
extends EditorTextureImportPlugin

func _load_image(source_file: String) -> Image:
    var image := Image.load_from_file(source_file)
    return image
    
func _get_import_options(path: String, preset_index: int) -> void:
    add_import_option("process/resize", Vector2i(0, 0))
    add_import_option("process/trim_alpha", false)
    add_import_option("process/invert_color", false)

func _pre_process(image: Image) -> Image:
    var resize: Vector2i = get_option_value("process/resize")
    var trim_alpha: bool = get_option_value("process/trim_alpha")
    var invert_color: bool = get_option_value("process/invert_color")

    if resize.x > 0 and resize.y > 0:
        image.resize(resize.x, resize.y, Image.INTERPOLATE_CUBIC)
    
    if trim_alpha:
        var rect := image.get_used_rect();
        var img := image.get_region(rect);
        image.set_data(rect.size.x, rect.size.y, image.has_mipmaps(), image.get_format(), img.get_data());

    if invert_color:
        for i in range(image.get_width()):
            for j in range(image.get_height()):
                var color := image.get_pixel(i, j);
                var inverted := Color(1 - color.r, 1 - color.g, 1 - color.b, color.a);
                image.set_pixel(i, j, inverted);
            
    return image

[beicause]

The naming of "add_import_option_advanced" is misleading. It made me think that it adds an option to advanced import.

However, for compatibility and consistency, I follow this naming as it's also used in EditorSceneFormatImporter and EditorScenePostImportPlugin.

[beicause]

Renamed EditorTextureImportPlugin to EditorTexturePostImportPlugin. For me it seems more similar to PostImportPlugin rather than EditorImportPlugin.

This PR has limitations. If you want to import custom resource that contains ctex, it is still only possible to use EditorImportPlugin and PortableCompressedTexture2D.

Let me know if this design is reasonable, or if there is a better way to extend texture importer.