Change non_null option to generate non-null accessors backed by nullable properties

[andrewparmet]

Replacement for #274. Instead of generating code with a non-null property that fails to deserialize, generate a nullable backing property (with the original name and the GeneratedProperty annotation) and a delegating non-nullable property with the prefix require.

Renames the non_null option to generate_non_null_accessor.

Code excerpts:

Basic usage of the option; declaring that a field ought to be present:

@GeneratedMessage("protokt.v1.testing.NonNullModel")
public class NonNullModel private constructor(
  @GeneratedProperty(1)
  public val nonNullStringValue: String?,
  public val unknownFields: UnknownFieldSet = UnknownFieldSet.empty()
) : AbstractMessage() {

  public val requireNonNullStringValue: String
    get() =
      requireNotNull(nonNullStringValue) {
        "nonNullStringValue is assumed non-null with (protokt.property).generate_non_null_accessor but was null"
      }

Implementing an interface by a delegate (note that the overridden property narrows the type of id to String):

@GeneratedMessage("protokt.v1.testing.ImplementsWithDelegate")
public class ImplementsWithDelegate private constructor(
  @GeneratedProperty(1)
  public val modelTwo: ImplementsModel2?,
  public val unknownFields: UnknownFieldSet = UnknownFieldSet.empty()
) : AbstractMessage(),
  IModel2 {
  override val id: String
    get() = requireModelTwo.id

  public val requireModelTwo: ImplementsModel2
    get() =
      requireNotNull(modelTwo) { "modelTwo is assumed non-null with (protokt.property).generate_non_null_accessor but was null" }