Bug#1099018: apt: provide HTTP headers on checksum mismatch (and other failures)

[Chris Hofstaedtler <zeha@debian.org>]

Package: apt
Version: 2.9.30

While investigating a checksum mismatch error today, DSA and me
would have had a much easier time if APT would print the received
HTTP headers on such an error.

IOW:

When printing...

 E: Failed to fetch http://deb.debian.org/debian/pool/main/p/pyjwt/pyjwt_2.10.1-2.dsc  File has unexpected size (24636 != 2390). Mirror sync in progress? [IP: 199.232.18.132 80]
    Hashes of expected file:
     - SHA256:18c7ac34d689629fef29f06a3de41a4c998c2a4ee42f9c36d7ebcaa12e051e8c
     - Filesize:2390 [weak]
     - MD5Sum:1dd7eb9413a1831538d87c7a1627d266 [weak]

..., please also print all received HTTP headers (including values),
for example (but not limited to) X-Served-By, X-Cache, X-Cache-Hits,
Age, Via, Last-Modified, Content-Length, Date.

Thanks,
Chris