Add mechanism for setting default ContextAttributes for ObjectMapper

[cowtowncoder]

Currently (2.12) the only way to specify default ContextAttributes for ObjectMapper is to directly modify DeserializationConfig/SerializationConfig. But it'd seem better to allow doing this via MapperBuilder, adding something like

defaultAttributes(ContextAttributes)

method (and simple tests).

[cowtowncoder]

Addition using MapperBuilder.defaultAttributes(ContextAttributes) (for 2.13 and 3.0); 2.13 also has direct ObjectMapper.setDefaultAttributes(...) since it is required for builder to work.