Add `IonObjectMapper.builderForBinaryWriters()` / `.builderforTextualWriters()` convenience methods

[cowtowncoder]

Currently IonFactory has methods for specifying whether binary- or textual-output should be specified. While this can be used via IonObjectMapper by specifying builder for IonFactory, this is bit unwieldy:

ObjectMapper mapper = IonObjectMapper.builder(
  IonFactory.builderForBinaryWriters().build()
)
// possibly configure and
.build();

and it'd make sense to add short-cuts to allow:

ObjectMapper mapper = IonObjectMapper.builderForBinaryWriters()
   .build();

Should also add Javadocs for existing builder() methods to indicate that the default is to use binary writers.

[cowtowncoder]

As per discussion, added in 2.12.2 as it's bit of a "special" release anyway :)