Prior to this change, the Spring MVC auto-configuration would add a new
formatter to convert `java.util.Date` to/from `String` using the
configured configuration property `spring.mvc.date-format`.

This commit adds a new `WebConversionService` class that registers
date formatters with a custom date format, or register the default ones
if no custom configuration is provided.
This avoids duplicating equivalent formatters in the registry.

With this change, date types from `java.util`, `org.joda.time` and
`java.time` are now all supported.

This commit also replicates this feature for WebFlux applications by
adding a new `spring.webflux.date-format` configuration property.

Closes gh-5523
Closes gh-11402