Add new JsonReadFeature for handling Mongo/BSON wrapper JSON extensions

[cowtowncoder]

Since Mongo(DB) defines a set of extensions ("shell mode" -- see https://docs.mongodb.com/manual/reference/mongodb-extended-json/), and since support for reading such "enhanced" content seems sparse, it may make sense to add optional support for handling at least most commonly used wrapper extensions.

Note: this does NOT mean reading actual binary BSON content, for which separate backend is needed: there already exists bson4jackson.

NOTE: this requires implmenetation of #467 as well, so that we avoid adding this as JsonParser.Feature as it only applies to JSON backend, not for other formats.

[cowtowncoder]

Actually, after thinking more about this, I think better way to do this would be to add a new backend in jackson-dataformats-text. This because there are possibly many configurable aspects and although reuse of much of json decoding functionality makes sense, adding Mongo/BSON exclusions in core json handler is probably not a good idea wrt maintainability.

[cowtowncoder]

No current plans, closing.