[agw] Replace C UE context structs with proto-generated C++ objects as values

[ardzoht]

Description

• For completing the Stateless migration to C++, hashtable OAI lib usage should be deprecated and removed.
• Currently for UE state, the hashtable values are represented by pointers to the C structs.
• Since we're aiming to use Protobuf::map, the C structs objects that are eventually serialized should also be migrated to directly the Protobuf message objects instead.

Goal

1. Replace C structs with Protobuf and List structure usage in SPGW task
2. Replace C structs with Protobuf in S1AP task
3. Replace C structs with Protobuf and List (for queue usages: e.g. common_proc) structure usage in MME_APP task
4. Revise the protobuf structures to use Enum instead of uint primitive type

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