feat: package-wide configure_logging / LOG_LEVEL

[eliteprox]

Summary

• Add configure_logging() and LOG_LEVEL env support for the livepeer_gateway package logger
• Call configure_logging() on package import (sets package level; adds a root handler only if none exist)

Test plan

• LOG_LEVEL=DEBUG with examples/write_frames.py shows livepeer_gateway.* logs with no example-side logging setup
• Importing the package in an app that already configured logging does not replace root handlers

[j0sh]

• I would not add a library-wide env var, see feat(live-runner): env-var fallback for register_runner provisioning fields #34 (comment)
• Don't call this during package import, because it breaks later application calls to logging.basicConfig(). Let apps invoke the configuration handler on their own, if they want.
• Let's keep the default BASIC_FORMAT of "%(levelname)s:%(name)s:%(message)s" (see where its used in other examples) and make this configurable by the app
• Maybe name the file logging.py or thereabouts

Codex also suggested:

invalid level names silently become INFO, while getattr(logging, ...) can resolve attributes that are not logging levels. It should validate against logging.getLevelNamesMapping() and raise ValueError