CI: initial commit of CI

[TomVasile]

Changes:

• adds ci for pull_request and merge (push) to master
  • runs fmt, audit, clippy, test, and bench
  • fails on warnings found

Ticket: https://app.zenhub.com/workspaces/engineering-l1-60953fafb1945f0011a3592d/issues/casper-network/governance/254