Use an intermediate animation importer

[reduz]

• Animation importer now goes through an intermediate class
• Allows centralizing all animation compression and optimization
• Allows better access to animation data by scene importer plugins

This work is WIP

[fire]

1. Lyuma: Can I ask what the Vector3::AXIS_X argument means when adding blend shape tracks?

animation->node_track_add_axis_key(track_idx, ImporterAnimation::TRACK_TYPE_BLEND_SHAPE_KEY_0 + weight_i, Vector3::AXIS_X, t, b, 0, a, 0, c);

📛 reduz 📛: Lyuma blend shapes just use the X axis, ignore the others

Lyuma: does it store 0 for the other axes?

📛 reduz 📛: yeah, well more like nothing is stored

I guess it's just an in memory format so it doesn't matter if it stores unused axes?

📛 reduz 📛 yes internally nothing is stored if the channel is not used

2. Lyuma: might suggest to rename the struct Node to a more unique name that isn't a widely used base type.

3. Fire: Do we want to handle the case of non-equally spaced quaternions? This can be solved by quaternion squad or others. Squad is defined here https://github.com/KhronosGroup/glTF/blob/b6a148cf742efd8e1e1bb0457e6e2a5989b1c387/extensions/2.0/Vendor/EXT_animation_sqlerp/README.md#cubicslerp-interpolation

[reduz]

Closed in favor of #53689