Improve local storage of assets & cached image files

[michaelbromley]

Is your feature request related to a problem? Please describe.
Currently, when using the AssetServerPlugin, it will use the LocalAssetStorageStrategy which is part of Vendure core. This storage strategy dumps all assets into a single directory.

For a very large number of files, this approach can have performance implications - thousands of files in a directory can be slower to read.

Describe the solution you'd like
We can use an approach known as a "hashed directory structure" in which a hash is made from the filename (e.g. md5) and then this hash is used to place the file in a sub-directory structure according to the first part of the hash. This is e.g. how git stores files in its /objects directory.

A reasonable way to store a small multitude of files on a file system is through a hashed directory structure with details on the files kept in a database table. One advantage of this method is that it keeps directory reads and file access times quick by not putting too many files into a single directory. source

I propose the following:

1. Create a new HashedDirectoryStorageStrategy
2. Make the storage strategy in the AssetServerPlugin configurable. This allows users to continue using the existing LocalAssetStorageStrategy to avoid breaking existing projects.
3. Set the new HashedDirectoryStorageStrategy as the default in the AssetServerPlugin.
4. Use the same approach for storing cached images - this can be hard-coded as it is an internal implementation detail of the AssetServerPlugin.