feat: add cast nullability migration path.

[jquense]

Adds a cast option to not throw on optionality mismatches after casting. This is being added to support migration from pre-v1 where cases like string().nullable().required() are supported. This case is generally used to handle parsing server side "default" values to feed into a form. Initial values may be null or missing but require filling in by a user before submitting the value back to server. cast(value, { assert: 'ignore-optionality'}) will only throw if the cast value doesn't match the strict type, but will pass for null or undefined values. The returned TS type will also be optional even if the current config would create a present type, just to be safe.

Going forward the approved way to handle these cases is with multiple schema using conditional when's to set nullable or using schema.deepPartial() to create an optional version of itself.