Add support for basic "is-required" checks on deserialization using `@JsonProperty(required=true)`

(note: moved from https://jira.codehaus.org/browse/JACKSON-767)

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As per title, although full validation of values is out-of-scope for Jackson, very basic “must be included in JSON” checks may fall in 80/20 line of things. Especially since we already have annotation to use.

The trick here is implementation; from two distinct parts:

1. This requires handling and retaining of extra state (what has been included) during deserialization by BeanDeserializer, and passing it; not trivial to do with current code.
2. Due to combinatorial explosion by features, efficiency, we need to be careful not to add Yet Another combination of things to tackle, requiring adding yet more similar (but not identical) methods

[cowtowncoder]

@utkarsh2012 If there is work we will update the issue. There is not much point in asking for things when there is no indication anything has changed. If you want to ask questions, in general, mailing list is a better place.

[utsengar]

Do we have an update on this for class attributes? Been 1yr since last update.