providers should download small files concurrently

[willmurphyscode]

What would you like to be added:

Right now, Vunnel spends a long time running for providers that download many small files, because it downloads these files one at a time. Instead, Vunnel should download N files concurrently in order to spend less wall clock time blocked on network I/O.

Why is this needed:

It makes vunnel and grype-db easier to work on because providers will finish faster.

Additional context:

Example of provider downloading small files one at a time: https://github.com/anchore/vunnel/blob/main/src/vunnel/providers/amazon/parser.py#L115