Enhance Logging Configuration with Environment Variable Support

Closes #64

Description This update introduces support for setting log levels via the LOG_LEVEL environment variable.

Key Changes:

Environment Variable: Log level can now be set with the LOG_LEVEL environment variable. Defaults to 'info' if unset.
LogLevel Enum: New LogLevel enum class for clear log level representation.
String to Enum Conversion: Converts log level strings to LogLevel enums.
spdlog Integration: Converts LogLevel enums to spdlog log levels.

cyrusbehr / tensorrt-cpp-api