Improve conversion from JSONB Postgres type to hive.operation

Instead of converting JSONB -> string -> fc::json -> hive.operation, implement direct conversion.

This also introduces functions for Postgres/c++ interop:

• pg_call_cxx: to be used in Postgres functions implemented as C functions when they need to call c++ code
• cxx_call_pg: when c++ code needs to call Postgres function

These function are needed because Postgres uses longjmp for error handling, which doesn't play well with c++ exceptions. They take care of translating Postgres errors from/to c++ exceptions as needed.