Support NOT NULL columns #149
Description
At the very least, explicitly via the @Column annotation.
For better ergonomics, also allow inferring of non-nullability from standard non-nullability annotations (old javax.annotation.{Nonnull,ParametersAreNonnullByDefault} and org.jetbrains.NotNull; new org.jspecify.{NotNull,NullMarked}). This will be probably optional, enabled by a Java system property.
The default column nullability should be NULL, except possibly for the primary keys, where it's strongly advised by YDB developers to be NOT NULL (we'll switch the defaults via an explicit system property now, maybe changing it in YOJ 3.0.0?)