Add metadata to resource previews

[KoBeWi]

Prerequisite for #63263

This PR adds preview metadata to EditorResourcePreview. A metadata can be image dimensions, audio stream length, model triangle count etc. In #63263 I want to display this information in a tooltip, but the problem is that fetching this information requires loading the resource, which can be costly sometimes. This PR allows to easily retrieve and cache the metadata, because generators already load the resource to make the preview.

What I do is pass a HashMap to the generate() method of EditorResourcePreviewGenerator and the generator can add any metadata to that hashmap. It can be later retrieved by using resource path + metadata name. For now I only added "dimensions" metadata for textures.

Marking as draft because:

• this metadata is not saved yet
• the PR should force preview cache invalidation, because old cache obviously doesn't have the metadata
• I think I should pass Dictionary instead of HashMap, because I convert it anyway 🤔
  Any advice on these welcome, but I'll try to figure it out myself.

[KoBeWi]

This is just a bonus refactoring.

[KoBeWi]

Ok this is probably ready to review. The metadata is correctly saved and loaded. Not sure how to force cache invalidation though; old previews don't have metadata stored.

[akien-mga]

Seems good to me. Lacking @reduz's explicit go but it doesn't seem too critical, he can validate later if needed.

Edit: Discussed it with reduz:

I am just not happy with the approach in general, but thinking how to do this properly is a good amount of work
we can probably still merge it anyway
If its important, it can be merged as-is, its fine

Needs a rebase. Might also be good to clarify what [param metadata] is used for in the docs.

[KoBeWi]

Rebased. I clarified the metadata, but right now it can't be used from a plugin. Maybe I'll rework #63263 to add EditorResourceFileTooltipPlugin as suggested by reduz.

[akien-mga]

Needs another rebase to fix CI.

[akien-mga]

I promise the CI failure isn't my fault this time :P

[akien-mga]

Thanks!

[aaronfranke]

Including the .cpp here causes a conflict with #70080 which also included the .cpp for the tests. I opened #78108 to make a header for this file so we can stop including the .cpp file.

[Zylann]

The new required argument breaks compatibility, my plugin that was working from Godot 4.0 is no longer working in 4.1:

Parse Error: The function signature doesn't match the parent. Parent signature is "_generate(Resource, Vector2i, Dictionary) -> Texture2D".

And if I fix this, it won't work in 4.0 anymore.