Those will make it possible to securely host 'kopia server' embedded
in a desktop app that runs in the background and can access UI.
- added support for using and generating TLS certificates
- added /api/v1/shutdown API to remotely trigger server shutdown
- added support for automatically shutting down server if no requests
arrive in certain amount of time
- added support for generating and printing random password to STDERR
TLS supports 3 modes:
1. serve TLS using externally-provided cert/key PEM files
2. generate & write PEM files, then serve TLS using them
3. generate and use emphemeral cert/key (prints SHA256 fingerprint)
