[SRU] command-not-found metadata support

[Impact]

 * Starting with Ubuntu Focal (20.04), the APT client expects APT sources to provide command-not-found (cnf) metadata files.

 * This package provides package mirrors, however does not support mirroring this cnf metadata up until upstream version 1:2.32, which is later than the version in Focal (20.04) and Groovy (20.10).

 * This upload cherry-picks upstream commits 292e9e9c, 39055366 and 9606535d, in order to add the cnf metadata support.

[Test Case]

 * Create a mirror of e.g. `bionic main` and `focal main`: debmirror -d bionic,focal -s main --passive --keyring=/usr/share/keyrings/ubuntu-archive-keyring.gpg --progress /opt/debmirror

 * Serve the root directory of /opt/debmirror as webroot.

 * Point APT `sources.list` at this webroot: deb http://mirror.example.com/ubuntu/ focal main

 * Attempt to run `apt update` on Focal (20.04) client to see that it fails to obtain the cnf metadata.

 * Optionally run `apt update` on Bionic (18.04) client to see that it works fine without the cnf metadata.

[Where problems could occur]

 * New cnf files are expected to be created and maintained by the debmirror package. The regression risk for incorrectly downloading these files does not serve more harm than not downloading the files whatsoever (current state of 20.04 release), as the APT client does not use the mirror in either case.

 * The change as uploaded was released in upstream as part of 1:2.32, tagged 23 Mar 2019, and is part of Debian stable. This serves as evidence of it having had testing carried out over time.

[Other Info]