[5.x] Improve AssetFolderPolicy performance

[jesseleite]

In our AssetFolderPolicy, we are currently performing a whole bunch of redundant ability checks, which really slows down performance in super large asset containers.

For example, this ability check is done in our AssetFolderPolicy:

But then right after that, you can see we loop over every asset in the folder:

Which essentially just does the same ability check within AssetPolicy:

This PR changes improves performance by only looping over each asset if a custom AssetPolicy class is registered. If a custom policy is registered, there is likely extra logic that run to be checked on an asset-by-asset basis, so we'll do the more thorough check only in those scenarios.

Fixes #9710