refact(pcf): Made configuration values private and read-only

src/common/config/config.cpp

src/common/config/config_types.cpp

src/common/config/config_types.hpp

src/common/config/config_yaml_file.cpp

src/common/config/config_yaml_file.hpp

src/oai_pcf/main.cpp

src/pcf_app/pcf_client.cpp

src/pcf_app/pcf_nrf.cpp