[Nix] Auto-generate url json configuration files
Summary:
Instead of users having to write this manually, just create it.
https://linear.app/comm/issue/ENG-1936
Test Plan:
# move old directory
test -d keyserver/facts && mv keyserver/facts{,.bak}
nix develop
# assert files were written
cat keyserver/facts/{commapp,landing}_url.json
# move old directory back
test -d keyserver/facts.bak && rm -r keyserver/facts && mv keyserver/facts{.bak,}Reviewers: ashoat, atul, abosh, varun, O1 Nix
Reviewed By: ashoat, O1 Nix
Subscribers: tomek
Differential Revision: https://phab.comm.dev/D5274