BOSTON — Gov. Charlie Baker is unveiling a plan he says will help encourage the development of unused or underused state-owned properties into new opportunities for affordable or market-rate housing, economic development, or recreational open space.Baker said Monday that as one of Massachusetts' largest property owners, state government has an opportunity to use its underutilized real estate to build housing and conserve open space, while driving economic growth.Baker pointed to more than 40 initial properties the state is seeking to develop.