Provides a simple caching mechanism for TRUD files.

This is deliberately not a generic download manager. It provides only
a lightweight and synchronous cache for downloads.

Check provides support for validating downloaded TRUD files.

There are four mechanisms for checking the integrity of downloaded files.

1. SHA256 hash provided in release metadata
2. Checksum file containing a hash (legacy; using FCIV)
3. Signature file (GPG)
4. The use of https to download the files.

It appears as if the legacy checksums are generated using a Windows command
line tool called FCIV (https://docs.microsoft.com/en-us/troubleshoot/windows-server/windows-security/fciv-availability-and-description).

<?XML version="1.0" encoding="utf-8"?>
<FCIV>
    <FILE_ENTRY>
       <name>ntdll.dll</name> <MD5>bL/ZGbqnyeA8hHGuTY+LsA==</MD5>
    </FILE_ENTRY>
</FCIV>

It appears that FCIV generates only MD5 and SHA1 hashes.

TRUD has been updated (see https://isd.digital.nhs.uk/trud/users/guest/filters/0/releases-help/sha256)
and now recommends using the SHA256 hash provided in the release metadata,
and so we can now avoid using the FCIV based hash system entirely in favour
of simply checking the SHA256 hash.

Support for the UK NHS Digital's TRUD.
TRUD is the Technology Reference data Update Distribution.