Provide package file download api

[jbonnet]

All the logic is apparently available, we're only missing it in the API

[jbonnet]

@dang03, @shuaibsiddiqui, @felipevicens
Since packages can be big files, I was thinking of connecting directly from the API to the Catalogue (i.e., without using the Package micro-service); what do you think?

This would save the time for opening/saving the package twice, first in the API (needs to validate owner, format, etc.) and second in the Package micro-service.

[jbonnet]

@dang03, @shuaibsiddiqui, @felipevicens
...and we should improve later 'cause we have currently 3 calls:

1. call to get package's meta-data (gives the son_package_uuid)
2. call to get package file's name
3. call to get the package file

At least 1. and 2. could be merged, no? Or am I missing something?