Support OSLS as a Serverless peer

[mnapoli]

Summary

• Reapplies the dependency update from move dep to open source serverless #425 on top of current master.
• Uses OSLS for local development/builds while keeping both OSLS and Serverless Framework as optional peers.
• Updates the unit-test harness and CI matrix to exercise both osls and serverless@3.

Test plan

• npm run check-format
• npm run lint
• npm run type
• npm test -- --runInBand
• SERVERLESS_TEST_PACKAGE=serverless npm test -- --runInBand after temporarily installing serverless@3.40.0

Replaces #425.

[dan-greene-brivo]

Thanks for picking up my dropped baton!!!