Metal: Implement `texture_create_from_extension`

[kroketio]

This PR implements texture_create_from_extension() for Metal.

The Vulkan device driver implements texture_create_from_extension() to handle externally backed textures (OpenXR module, or otherwise), which takes an incoming VkImage, creates a view, and returns it as TextureID.

https://github.com/godotengine/godot/blob/e4e024ab88efe74677769395886bc1b09eccbac7/drivers/vulkan/rendering_device_driver_vulkan.cpp#L1745-L1750

For Metal, there is no exact equivalent to Vulkan's VkImageView, so for this driver we only need to cast the incoming handle to id<MTLTexture>, then return as TextureID.

Parameters p_type, p_format, p_array_layers, p_depth_stencil are ignored - MTLTexture (and the callee) already have this information and is only relevant when reinterpreting or remaping the texture in different ways.

The use of __bridge with void* in the casting is to signal we are not transferring ownership, and is a safe cast to bridge from C-style memory to Objective-C memory management (but correct me if I'm wrong).

loosely related to #96860