Provide methods for inspection of JsonObjectBuilder

At times, it may be useful to inspect a JsonObjectBuilder without needing to build it (such as determining whether or not the object contains a particular key, for example). Methods could be provided on JsonObjectBuilder to examine the state of the underlying JsonObject to be built

Start by reviewing the JsonObjectBuilder and JsonObject APIs to understand the existing builder and inspection capabilities. The work is complete when the required inspection methods are agreed on and exposed on JsonObjectBuilder without requiring the object to be built.

Written by the indexing model from the issue text.