Separate required from nullability

[almson]

Validators like the Helm validator make a distinction between required and nullable properties. This makes sense.

This commit:

• Allows primitive properties to be optional (they may have default values, eg when using @Jacksonized @Builder)
• Allows nullable properties to be required (why not?)
• Looks for javax.annotation.Nullable and org.jetbrains.annotations.Nullable properties
• Adds config option to treat all reference properties as nullable
• If above is true, looks for javax.annotation.Nonnull and related javax.validation.constraints.

[jandree-netflix]

@mbknor can this get merged and a new version released with this change? It would be really valuable to have as way to override behavior of primitive properties.

[almson]

@jandree-netflix This is already implemented in my Java-based fork. If there's interest, then I'd love to maintain it.