Infer required properties from constraint annotations

[joffrey-bion]

The fact that some properties are "required" or "optional" could be inferred from constraint annotations like the following:

• javax.annotations @Nonnull/@Nullable
• JetBrains' @NotNull/@Nullable
• Hibernate's validation annotations like @NotBlank/@NotEmpty.

[joffrey-bion]

JetBrains's @NotNull/@Nullable are not available at runtime, so these ones cannot be taken into account.

[jpeffer]

Would be useful to support custom constraint annotations somehow as well.