Use zerolog for logging #771

Add proper logging to the desktop client. Splitted from !172

Changes

Multiple log levels are supported and can be set via env variable (LOG_LEVEL)

• trace (LOG_LEVEL=TRACE)
• debug (LOG_LEVEL=DEBUG)
• everything else: INFO

Comments/to discuss/TODOs

• TODO: Update README
• I can not build on Linux/Windows => please check
• running golangci-lint run --path-prefix=./... --timeout=5m shows a lot of things to improve (that's a seperate issue) (Update: most of them are fixed in !186 (merged))
• I like the output of zerolog, but the written log file itself is a line based, so not super easy to read (I used zerolog because it was used in bitmask-core)
• Both logger (log and zerolog) append their logs to a single log file. Currently, the log file is never closed. I will look at it...
• What do you think about the time format: 2024-04-23T15:53:10+02:00
• There is also some qml logging printed to stderr: I don't like the big logs (the big json)
  • The go code now logs to stdout, so that I can redirect stderr to /dev/null

Some ideas about log levels

• fatal: used for non-recoverable errors, terminates whole application (just used fatal if it was used before):
  • backend/auth.go -> generateAuthToken -> writeFileToDisk
  • pkg/backend/actions.go => startVPN
  • pkg/backend/api.go => RefreshContext
• warn: an error occured, but we can continue/recover from that
• info: a relevant information to the user/something happend
• debug: useful for dev debugging/analyzing censored environment
• trace: use to show which functions are called (can be nice for dev, e. g. "stop the openvpn" is called twice sometimes during teardown, also RefreshContext calls a lot of stuff in the background (toJson())