Based on conversion work feedback, this makes two changes to specialized server parameters:

1. Allow defining a default value without any bound data.  In this scenario, the class definition will have a {classname}:kDataDefault constexpr, but it will be up to the implementing callsite to make use of it.
2. Default implementations of ServerParameter::set() will now accept any BSONElement which is coercible to a std::string, rather than just the String type only.  This is useful for numeric specialized server parameters as number can easily round-trip through strings.