Generate images for arm64-rpi with Packer

[openoms]

Related to the work of @hashes4merkle in https://github.com/rootzoll/raspiblitz/issues/2842 and discussed in https://github.com/rootzoll/raspiblitz/issues/3053

fixes #91

• [x] building the arm64-rpi image in docker with https://github.com/mkaczanowski/packer-builder-arm
• [x] moved the packer scripts out of the .github directory
• [x] using HCL2 syntax
• [x] apt-get everywhere to fix warnings re. apt
• [x] nameservers added to fix networks errors
• [x] force progress bard for wget to shorten logs
• [x] generating sha256 checksum with the post-processor
• [x] aiming for a virtualbox image for amd64 - which can be converted to a raw image
• [x] arm64-rpi image tested
• [x] documentation update