cli: add support for reading a file content from stdin in 'swh identify' command
This allows for example to type:
curl -s https://archive.softwareheritage.org/browse/content/sha1_git:64582b78792cd6c2d67d35da5a11bb80886a6409/raw/ | swh identify swh:1:cnt:64582b78792cd6c2d67d35da5a11bb80886a6409 -