[agw] Convert GUTI struct into string representation

[ardzoht]

Description

• Currently the map.h interface under lte/gateway/c/core/oai that wraps up most of the OAI C++ standard hashmap implementation uses std::unordered_map under the hood.
• In terms of the broader C++ hashtable migration, we want to use a common standardization of Protobuf data structures and objects to allocate the data.
• The only caveat to this is that Protobuf::map does not support complex key lookups. The GUTI mapping (which holds guti_t as object, and mme_ue_id as key) is the only complex key usage in the OAI codebase.

Goal

• guti_t struct usages should be revised to use a string representation on all OAI codebase after reading the GUTI info on the MME procedures.

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