Add MD5 checksum check when downloading package

Currently there is no checksum performed on a package downloaded by Nuxeo from Connect.

Connect client should:

1. download the package
2. compute an MD5 checksum of the downloaded file
3. compare this sum to the digest property

All is here to do so:

1. on Connect side there is the digest property containing a MD5 checksum of the attached package
2. in the JAVA APIs it is also available:
   https://docs.oracle.com/javase/8/docs/api/java/security/MessageDigest.html

a practical example:
https://www.mkyong.com/java/java-md5-hashing-example/