Add support for setting log level from env variable

[tadeas-hejnic]

Is your feature request related to a problem? Please describe.
Add support for setting log level from env variable.

Describe the solution you'd like

void setLogLevelFromEnv(const std::string& envKey = "AYON_USD_RESOLVER_LOG_LVL", bool applyToFile = false) {
        auto envVal = std::getenv(envKey.c_str());
        if (envVal) {
            setLogLevel(std::string(envVal), applyToFile);
        }
    }