Add "maybe" operations to JSON.

[BillyONeal]

Extracted from #1514

This avoids some check/confirm anti-patterns and discourages null-dereference prone code like *example.get(). Example:

vcpkg-tool/src/vcpkg/commands.set-installed.cpp

Line 62 in ad7b71c

job.insert(JsonIdId, Json::Value::string(*args.github_run_id.get()));