Design support for tuple extension features in the connector

[bigbes]

Three protocol feature flags are defined in go-iproto but not yet implemented in the client’s handshake or serialization logic:

• IPROTO_FEATURE_DML_TUPLE_EXTENSION — enables binary tuple encoding (MP_EXT type 7) in DML responses (e.g., SELECT, INSERT)
• IPROTO_FEATURE_CALL_RET_TUPLE_EXTENSION — enables MP_EXT tuple encoding in return values of CALL/EVAL
• IPROTO_FEATURE_CALL_ARG_TUPLE_EXTENSION — enables MP_EXT tuple encoding in arguments of CALL/EVAL

These flags are declared in feature.go but are not advertised during connection handshake in go-tarantool (see protocol.go), and the corresponding MP_EXT encoding/decoding logic is missing. Parsing of IPROTO_TUPLE_FORMATS is also missing, with mapping of fields to their respective names in format.

Related to:

• #467
• #362