Support username/password authentication for PrivateMirror

[candrews]

Describe the bug

PrivateMirror currently only supports making unauthenticated requests. It would be really great to support authentication analogous to the support ClamAV already has for HTTP proxies provided by the existing HTTPProxyUsername and HTTPProxyPassword options.

Perhaps these new configuration options could be called PrivateMirrorUsername and PrivateMirrorPassword?

How to reproduce the problem

echo "PrivateMirror https://clamav-mirror.example.com" >> /etc/clamav/freshclam.conf
freshclam

is how a Private Mirror is used today.

It would be nice to be able to set up authentication for the use of the Private Mirror, like so:

echo "PrivateMirror https://clamav-mirror.example.com" >> /etc/clamav/freshclam.conf
echo "PrivateMirrorUsername clamavuser" >> /etc/clamav/freshclam.conf
echo "PrivateMirrorPassword clamavpassword" >> /etc/clamav/freshclam.conf
freshclam

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