Preserve asset data from fields when running statamic:assets:meta generation command

[ncla]

Fixes #4595

The fix in the issue description works fine. It also affects focus being saved as it goes under data property too.

I thought that generateMeta method in Asset class already accounts for existing asset data and merges it together here:

https://github.com/ncla/cms/blob/3ed1e592b309f34379db07fb95af406b97fe87f8/src/Assets/Asset.php#L235-L253

But with line $meta = ['data' => $this->data->all()];, as far as I understood it is for use cases where you are creating Asset object manually? So as that is not the case here, we have to simply hydrate the asset, as AssetRepository::save() does not account for such situation.

It is an additional call to file system for generating meta. No matter how I look at it, the command will have to look up the meta file, unless we can pull this data from stache/cache or something?

I think most people are fine with this behavior (?) so there's no need to include a separate command flag like --without-hydration for disregarding existing data, or to make the command create less file system calls. I am just wondering as there's lately been some issues with the asset hydration, so just want to be considerate.

As bonus I have added tests to sleep better at night. Borrowed, inspired logic from other asset tests.