Add a storage abstraction layer

[gabiudrescu]

Add an abstraction layer to avoid being tied up to the local storage. FlySystem can hot swap local with s3 or any other kind of remote hosting for files by simply changing a configuration file.

[loic425]

@gabiudrescu We talked about your feature with @smildev at Mobizel and we think about creating a cookbook instead. Cause it's not a systematic need to use a remote hosting. We prefer to limit the amount of composer dependencies.

[gabiudrescu]

@loic425 your argument makes sense. the reason I think this should be in the app instead of a cookbook is because it will promote a way of thinking to those who use this application skeleton.

in my opinion, nowadays new apps should start from day 1 with being cloud ready in mind. and being tied to the filesystem of your webserver is not something you want in 2020, in the cloud environments we are working with today.

if the skeleton is working by design with a filesystem abstraction, the probability of people using this abstraction with a cloud ready file storage is increased, compared to the option of defaulting to a filesystem with no abstraction and creating some documentation about how to optionally activate such feature.

the reason I added this was to encourage the usage of better solutions for the long run. and flysystem doesn't feel to me it's a big dependency.

anyway, in the end, it's your call. if you don't think it is that useful, I have nothing against closing this PR and submitting another one with a cookbook.

though, if I am allowed, I would strongly suggest for implementing such an abstraction for the default way of handling filesystems in Monofony.

PS: one more reason to do this is to attract more attention by claiming Monofony is cloud ready and Amazon S3 compatible. Buzz words, but it can help grow the community by reaching to people with such needs.