get_url: Allow additional hashing algorithms

Allow all hashing algorithms that hashlib provides (currently 'md5', 'sha1', 'sha224', 'sha256', 'sha384', 'sha512'). Introduces a new parameter "checksum" with format: <algorithm>:<checksum>, e.g. checksum=md5:66dffb5228a211e61d6d7ef4a86f5758
Deprecates sha256sum. uses hashlib directly.


Also gives helpful error messages:
- msg: Could not hash with algorithm 'md6'. Available algorithms: ('md5', 'sha1', 'sha224', 'sha256', 'sha384', 'sha512')
- msg: The checksum parameter has to be in format <algorithm>:<checksum>

See also #6304