RCPP-61 Add BSON support

[leemaguire]

This PR adds back BSON functionality by bridging the BSON classes found in RealmCore.
usage:

        auto bson = realm::bsoncxx();

        bson = realm::bsoncxx((int32_t) 123);

        bson = realm::bsoncxx((int64_t) 123);

        bson = realm::bsoncxx(true);

        bson = realm::bsoncxx(123.456);

        bson = realm::bsoncxx(std::string("foo"));

        bson = realm::bsoncxx(std::vector<uint8_t>({0, 0, 0, 0}));

        bson = realm::bsoncxx(realm::bsoncxx::timestamp(100, 0));

        auto d = std::chrono::system_clock::now();
        bson = realm::bsoncxx(realm::bsoncxx::date(d));

        auto oid = realm::object_id::generate();
        bson = realm::bsoncxx(oid);

        bson = realm::bsoncxx(realm::decimal128(123.456));

        bson = realm::bsoncxx(realm::bsoncxx::regular_expression("[0-9]", "i"));

        bson = realm::bsoncxx(realm::bsoncxx::max_key());

        bson = realm::bsoncxx(realm::bsoncxx::min_key());

        bson = realm::bsoncxx(realm::bsoncxx::document({{"name", "foo"}}));

        bson = realm::bsoncxx(std::vector<realm::bsoncxx>({{"one", 123, true}}));

        bson = realm::bsoncxx(realm::uuid());