sd-json: add sd_json_build() wrapper macro that implies SD_JSON_BUILD_OBJECT()

sd-json: add sd_json_build() wrapper macro that implies SD_JSON_BUILD_OBJECT()

In 99% of uses of sd_json_build() we want to build an object as
outermost construct. Let's shorten this most common case a bit, by
adding sd_json_buildo() that implies this. This allows us to shorten
much of our code, all across the tree.