Webform validation handler

@alphex a question: should it be configurable per-handler or would using the site-wide configuration be enough for your use-case?

Asking because I'm not sure how to support translations inside the webform handler.

Amazing work, thank you! I love how much thought is going into this.

I had orignially envisaged the handler not even having form inputs for the validator settings if it was using the site defaults but passing the defaults through to the form elemetns could work too. Comes with pros and cons:

Pro: Webform settings won't be altered by changes to the site settings

Con: Webform settings won't be updated with changes to the site settings ;)

I can see use-cases for both.

Perhaps we could have a best-of-both-worlds approach? A checkbox to "override site defaults" which then displays the settings fields? That way if it is not checked, you just don't pass any config to the service when you run validate and it will use the auto-loaded site defaults.

Thoughts?

This is fantastic! Thank you for the work that went into it so far.

I've tested out the validator and it works brilliantly with the default settings (again, thank you), but it seems the option to override has an issue. The values get saved into the handler and it seems the checkbox to turn on a rule is working, but not the error message override.

Here's the settings I put the on the handler:

And here's the result:

Otherwise this is looking pretty complete, I'll make sure to credit you on the project page when this is committed as it's a big piece of functionality.

Looks great @joshua1234511, just one more comment on the code then I reckon we're good to go. I've tagged this for automated test but I won't hold back committing the feature waiting on those, they can be a follow-up if they're not done by the time the feature is ready.

I'll make a follow-up for tests. Thanks everyone, I think this is going to be a really useful thing :)

If anyone landing here would like to have a go at writing tests for this feature, please see #3274693: Write tests for webform validation handler