Add support to cache IPK packages for offline install

[meek2100]

No description provided.

[richb-hanover]

Hi! Thanks for this PR. I have a few thoughts/questions about it before I merge it. Specifically, I'd ask you to spend some time on the README to address the questions below:

• I don't quite understand the purpose... Why would someone use this? I see that this lets you do upgrades "off-line", but how that differ from just downloading the new firmware binary and the .ipk's to your laptop? Why is that an important use-case?

• Why not just use attended sysupgrade? What can this do that ASU won't? (I know that ASU requires an online connection...)

• How does this work in the face of apk that's coming soon? (24.10 continues to use opkg, but apk is coming in the next release.)

Thanks!

Rich