A moneyed Upper East Side family is looking to build an indoor pool in the cellar of their posh residence — but their neighbors aren’t exactly having a splash. Plans are in place — but work hasn’t yet begun — to construct a private underground swimming pool on a tony block near Fifth Avenue. The owners of the double-wide townhouse at 15-17 E. 77th St., which they bought in May 2023 for $30 million, have filed plans to gut-renovate and expand their luxury lair — whose total dollar estimate is