FlexRAN: New protobuf slice structure + Static Slice handling

openair2/ENB_APP/CONTROL_MODULES/MAC/flexran_agent_mac.c

openair2/ENB_APP/CONTROL_MODULES/MAC/flexran_agent_mac.h

.../ENB_APP/CONTROL_MODULES/MAC/flexran_agent_mac_internal.c

.../ENB_APP/CONTROL_MODULES/MAC/flexran_agent_mac_internal.h

openair2/ENB_APP/MESSAGES/V2/config_common.proto

openair2/ENB_APP/MESSAGES/V2/config_messages.proto

openair2/ENB_APP/flexran_agent_common.c

openair2/ENB_APP/flexran_agent_ran_api.c

openair2/ENB_APP/flexran_agent_ran_api.h

openair2/LAYER2/MAC/main.c

openair2/LAYER2/MAC/slicing/slicing.c