infra: add a command for downloading all corpora for a project from GCS (#2010)
infra: add a command for downloading all corpora for a project from GCS (#2010)

This partly addresses https://github.com/google/oss-fuzz/issues/2006.
FeliciaLim/oss-fuzz,skia-dev/oss-fuzz,google/oss-fuzz,google/oss-fuzz,skia-dev/oss-fuzz,google/oss-fuzz,robertswiecki/oss-fuzz,skia-dev/oss-fuzz,skia-dev/oss-fuzz,googlefonts/oss-fuzz,googlefonts/oss-fuzz,FeliciaLim/oss-fuzz,google/oss-fuzz,google/oss-fuzz,robertswiecki/oss-fuzz,googlefonts/oss-fuzz,robertswiecki/oss-fuzz,skia-dev/oss-fuzz,robertswiecki/oss-fuzz,robertswiecki/oss-fuzz,google/oss-fuzz,google/oss-fuzz,googlefonts/oss-fuzz,googlefonts/oss-fuzz,google/oss-fuzz,robertswiecki/oss-fuzz,google/oss-fuzz,FeliciaLim/oss-fuzz,FeliciaLim/oss-fuzz,robertswiecki/oss-fuzz,skia-dev/oss-fuzz,robertswiecki/oss-fuzz,google/oss-fuzz,skia-dev/oss-fuzz,skia-dev/oss-fuzz,google/oss-fuzz,skia-dev/oss-fuzz,googlefonts/oss-fuzz,FeliciaLim/oss-fuzz,skia-dev/oss-fuzz,robertswiecki/oss-fuzz,robertswiecki/oss-fuzz,skia-dev/oss-fuzz,googlefonts/oss-fuzz,googlefonts/oss-fuzz,googlefonts/oss-fuzz,googlefonts/oss-fuzz,robertswiecki/oss-fuzz