Add `WebMvcRegistrations` which can be used to provide custom
instances of `RequestMappingHandlerMapping`,
`RequestMappingHandlerAdapter` and `ExceptionHandlerExceptionResolver`.
Those instances are then used and processed by the Boot MVC
configuration.

Prior to this commit, developers could provide their custom instances
of MVC infrstructure components such as `RequestMappingHandlerMapping`
and `RequestMappingHandlerAdapter` only by using advanced configuration
strategies. Those advanced configurations involved subclassing
`WebMvcConfigurationSupport` which effectively turns off MVC
auto-configuration in Boot.

Fixes gh-5004
Closes gh-6100