azure-cli-extensions: transform set to json

This was done with the following cursed sed expression:

tail -n +3 extensions-generated.nix | sed -e 's/ = mkAzExtension rec / : /g' -e 's/=/:/g' -e 's/;/,/g' -e 's/\([a-zA-Z_][a-zA-Z_0-9-]*\) :/"\1":/g' -e '/^[[:space:]]*"description"/s/.$//' > extensions-generated.json

and some cleanup afterwards.

Signed-off-by: Paul Meyer <49727155+katexochen@users.noreply.github.com>