WIP - RUBY-3671 - Hash should honor BSON::Registry

[johnnyshields]

Fixes RUBY-3671

There is a BSON::Registry which determines how BSON is deserialized. For example, Mongoid has this code:

if allow_decimal128
  BSON::Registry.register(BSON::Decimal128::BSON_TYPE, BSON::Decimal128)
else
  BSON::Registry.register(BSON::Decimal128::BSON_TYPE, BigDecimal)
end 

However, BSON gem does not honor this registry for Hash, it always deserializes as BSON::Document, even when the registry is set to Hash.