Add a command to check files_managed integrity

[cristiroma]

It would be useful to have a robo command to check whether:

1. Files recorded in files_managed exists on the disk, and report those who do not exist
2. Files recorded in files_managed but are not recored in files_usage

./vendor/bin/robo files:integrity-check

M = File recorded in file_managed but missing from disk
O = File recorded in file_managed by no record in file_usage

FID	PATH	Problem	Count	Usage
123	public://uploads/test.pdf	M	15	taxonomy_term(s): 45, 46, node(s): 123, 1234
124	public://uploads/test.pdf	O	n/a
125	public://uploads/test.pdf	M, O	n/a