(2.8) Add new JsonValueFormat value BASE64

[cowtowncoder]

(note: see FasterXML/jackson-dataformat-avro#39 for background information)

Currently there is no good way to indicate type of byte[] encoded as Base64, since the most straight-forward type (from JSON perspective at least) of indicating String with format of base64 can not be used because of hard-coding of format values in JsonValueFormat enum.

While hard-coding itself is unfortunate and should be resolved somehow, for short-term needs addition of one more enum value makes sense.

[cowtowncoder]

Actually it looks like this can be implemented without additional value, by detecting type passed into expectArrayFormat() method, so will NOT add new type.