tools: support sha512 in the ICU current_ver.dep

[srl295]

• current_ver.dep has an md5 hash for the ICU version ( example: https://gist.github.com/srl295/752c86f0b7aaf45e15ca492715f13e19 )
• but, ICU has switched to SHA512
• support a key sha512 instead of md5 for verifying the download, otherwise the integrator has to calculate their own md5 which is not verifiable.

[srl295]

Something like this:

[
  {
    "url": "https://github.com/unicode-org/icu/releases/download/release-64-2/icu4c-64_2-src.tgz",
    "sha512": "5ecb4c230ba45918747a1cf9aef86f555aa07d5b29b1d07ab674e8013f46dfb907a0e9d6945db41155f9dc3012fd94e1152ffc19f61a68b6dfcbabdcb8ae9d78"
  }
]