get_url: Allow additional hashing algorithms

Allow all hashing algorithms that hashlib provides (e.g. 'md5', 'sha1', 'sha224', 'sha256', 'sha384', 'sha512'). Introduces a new parameter "checksum" with format: ```<algorithm>:<checksum>```, e.g. ```checksum="md5:66dffb5228a211e61d6d7ef4a86f5758"```
Deprecates parameter ```sha256sum```. Uses hashlib directly instead of methods in module_utils/basic.py.


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>```

Moved from https://github.com/ansible/ansible/pull/7639. I'm aware of the fact that my original PR was already denied. But I'm still convinced that this is the most future-proof solution as the possitibilities grow while hashlib grows. So I leave this here, open for discussion.