Implement getter functions for ETFeeder

[JoongunPark]

Summary

This PR updates ETFeeder to have getter functions of I/O attributes.
The I/O attributes include value/shape/type for the node.

Node that this feature is also required in other code in Feeder ( json_node.cpp json_node.h wrapper_node.cpp wrapper_node.h) which can be done after we decide details of JSON format.

Test Plan

This is tested with ASTRA-Sim as shown in below.

void Workload::initialize_process_group(std::shared_ptr<Chakra::ETFeederNode> node) {
    std::string pg_info = node->get_inputs_values();
    if (pg_info.empty()) {
        std::cerr << "Process Group Information is not encoded" << std::endl;
        return;
    }
    ....
}

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[tushar-krishna]

Looks good!