restructure SLOs, add new dropped-records SLO

slo-checker/dropped-records/Dockerfile

slo-checker/dropped-records/README.md

slo-checker/dropped-records/app/main.py

slo-checker/dropped-records/app/test.py

slo-checker/dropped-records/requirements.txt

...checker/dropped-records/resources/test-1-rep-success.json

slo-checker/record-lag/Dockerfile

slo-checker/record-lag/README.md

slo-checker/record-lag/app/main.py

slo-checker/record-lag/app/test.py

slo-checker/record-lag/app/trend_slope_computer.py

slo-checker/record-lag/requirements.txt

slo-checker/record-lag/resources/test-1-rep-success.json

slo-checker/record-lag/resources/test-3-rep-success.json