Add configurable log level support

[EyalAlphaEG]

I'm running Formance Ledger in production on GCP and couldn't find a way to configure the log level.
Currently, the ledger supports --debug for verbose logging and --json-formatting-logger for JSON format, but I wasn't able to find an option to set a specific log level (e.g., warning, error).
Having info as the baseline generates a high volume of logs in production, which increases cloud logging costs and makes it harder to identify actual issues.
It would be helpful to have a --log-level flag or LOG_LEVEL environment variable that accepts standard levels like debug, info, warning, and error. For example: ledger serve --log-level=warning or LOG_LEVEL=warning.
This is a common pattern in production services to reduce log noise while still capturing important warnings and errors. If this capability already exists and I missed it, I'd appreciate any pointers to the documentation.