Provide a callback interface for customizing WebMvc resource handler registrations

[filiphr]

Currently the WebMvc ResourceHandlerRegistrationCustomizer is not publicly exposed. However, the WebFlux one is.

Is it possible to align and expose the WebMvc ResourceHandlerRegistrationCustomizer publicly? The WebFlux one was exposed in #14587.

[gichanGim]

Hi, is this open for community contribution? If so, I'd be glad to take it on and mirror the WebFlux approach from #14587 on the WebMvc side, keeping the two aligned. Just let me know and I'll open a PR.