Allow easy activation of ISO-by-default date and time binding for Spring MVC

[odrotbohm]

As indicated in this ticket, it's a bit cumbersome (though doable) to make Spring MVC properly bind the ISO dates and times created through HTML input elements of type date or time. A custom WebMvcConfigurer can be used to enable this by default:

@Bean
WebMvcConfigurer isoDatesByDefault() {
  return new WebMvcConfigurer() {
    public void addFormatters(FormatterRegistry registry) {
      DateTimeFormatterRegistrar registrar = new DateTimeFormatterRegistrar();
      registrar.setUseIsoFormat(true);
      registrar.registerFormatters(registry);
    }
  }
}

As it's very likely to be what users expect to work by default, it would be cool if this could be enabled via a property. I guess for backwards compatibility reasons it should stay deactivated by default.

[wilkinsona]

Joining some dots. This was requested in #18511 alongside properties for configuring the time and date-time format. It was closed as being superseded by a pull request. The pull request doesn't add the ISO support, just the properties for time and date-time format.

There are two approaches that could be taken here:

1. Add a separate property for enabling ISO formatting. This would then result in the properties for the date, time, and date-time formats being ignored.
2. Allow a special value of iso (case-insensitive) to be used when configuring the date, time, and date-time formats.

There are pros and cons for each of these. The pro for 1 is that it gives you a single property to enable ISO formatting. The con is that it can lead to other properties being ignored. The pro for 2 is that no properties will be ignored as a result of another property being set. The con is that you need to set three properties to enable ISO formatting.

2 is our preferred option.