Add more _templated_ JsonUtils

[zadjii-msft]

In #2515 I started adding some really useful JsonUtils that let us get a property from a json object if it exists.

Unfortunately, I made them like this:

    void GetOptionalColor(const Json::Value& json,
                          std::string_view key,
                          std::optional<uint32_t>& target);

    void GetOptionalString(const Json::Value& json,
                           std::string_view key,
                           std::optional<std::wstring>& target);

    void GetOptionalGuid(const Json::Value& json,
                         std::string_view key,
                         std::optional<GUID>& target);

    void GetOptionalDouble(const Json::Value& json,
                           std::string_view key,

They really should have all been template specializations of GetOptionalValue, but I didn't want to burn a day trying to fight compiler errors.

Furthermore, we should probably also just have JsonUtils::GetValue<typename T>() that does the same thing, but w/o the optional<T>'s.

[DHowett-MSFT]

Thoughts: detail namespace? Default conversion function is detail::convert_from_json<T>?