Add built in support for validating options #227
Description
I just made an Option class that takes strings that are required to be a certain format. I would like the configuration to fail after hydration if the values provided are invalid. A good example is if you need an email (ignoring how hard it can be to validate emails).
Today I can solve this by creating an IPostConfigureOptions that resolves to a ValidateOptions class that checks if the options in question implement an IValidatable interface, and if so calls Validate().
It would, however, be preferable if I knew that validation always ran last (after all post configure handlers - if any). Therefore I suggest adding first class support for IValidateOptions which runs after runs after IPostConfigureOptions. This interface would behave the exact same as IPostConfigureOptions, except it would be guaranteed to run after all IPostConfigureOptions.
It can also be discussed whether or not default implementation that does something similar to my described ValidateOptions class would be useful.