add support for wildcard/patterns

Sometimes only a subset of files is needed when the user runs import or pull data from a data directory. It is convenient to define a file pattern for an import.

From https://discuss.dvc.org/t/working-with-a-small-subset-of-remote-data/541
Related: https://github.com/iterative/dvc/issues/4705, https://github.com/iterative/dvc/issues/4815

Patterns to implement:

• simple wildcard dvc pull cats-dogs/data/train/dogs/*.img
• whole wildcard dvc pull cats-dogs/data/train/{dogs,cats}/???.img
• globstar/ricursive dvc pull cats-dogs/data/train/**/*.img
• iterator dvc pull cats-dogs/data/train/dogs/%C.img?counter=1:100
• date dvc pull users/%Y/%m/%d/users.csv?startdata=2020-09-01,enddate=now,ignoremissing

The first three patterns should use a regular Unix file syntax. While the last two require a special language to define the pattern - we need to find a good examples.

Start by reviewing related issues 4705 and 4815 and the linked discussion about selecting subsets of remote data. Define the syntax and scope for the three Unix-style patterns and the proposed iterator and date patterns; done means the agreed patterns work with dvc pull and import, including the stated examples.