Add pretty-printing for structs, comparable to Rust's std::fmt::DebugStruct

[Databean]

Supports abseil string formatting, libfmt, and ostreams.

Example usage:

const PrettyStruct inner =
    PrettyStruct("Inner").Member("i1", 1).Member("i2", 2);
const PrettyStruct outer =
    PrettyStruct("Outer").Member("o1", inner).Member("o2", inner);

formats as

Outer {
  o1: Inner {
    i1: 1,
    i2: 2
  },
  o2: Inner {
    i1: 1,
    i2: 2
  }
}

Bug: b/474678754