realtime: auto-select raspi variant when appropriate

[orndorffgrant]

Why is this needed?

This helps users get the right realtime-kernel when running on a raspberry pi. It also informs users of what variant will be installed when it is auto-selected.

Test Steps

Ideally test on a rpi3/4/5 and test that auto-selection works as expected (select raspi on 4/5 and generic everwhere else) Or you can trick the rpi5 detection on a multipass vm with something like this:

./tools/test-in-multipass.sh noble

# now in vm
pro attach --no-auto-enable

cat > /usr/local/bin/dpkg << EOF
#!/usr/bin/bash
echo arm64
EOF
chmod +x /usr/local/bin/dpkg

sed -i 's/(proc_file_path)/("\/home\/ubuntu\/fake")/' /usr/lib/python3/dist-packages/uaclient/entitlements/realtime.py
echo "Raspberry Pi 5 Model B Rev 1.0" > /home/ubuntu/fake

Then pro enable realtime-kernel and ensure that the raspi variant is auto-selected.

Also test that raspi is not selected if the proc file is mocked but the arch is not arm64.

Checklist

• [x] I have updated or added any unit tests accordingly
• [ ] I have updated or added any integration tests accordingly
• [ ] Changes here need to be documented, and this was done in:

Does this PR require extra reviews?

• [ ] Yes
• [x] No

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