Add configuration propertie media types (reactive) with Mustache views.

[polarbear567]

For issue 28858.
Now I just add media types, and here are some questions:

1. content-type is supported as spring.mustache.content-type now, do we need to migrate to servlet?
2. Does freemaker need the same change?
   Because these problems are uncertain, the current changes may not be in the final code location.
   Hi @wilkinsona , can you help to check?

[wilkinsona]

content-type is supported as spring.mustache.content-type now, do we need to migrate to servlet?

Yes, it's servlet-specific so that should be made clear by the property name. This is quite a big change as it means that MustacheProperties can no longer be a sub-class of AbstractTemplateViewResolverProperties.

Does freemaker need the same change?

No. FreeMarker is only supported with servlets so there's no need to distinguish between servlet-specific and reactive-specific properties.

[wilkinsona]

Thanks for the PR here, @polarbear567. Looking more closely at this, it's become apparent that there's quite a bit more work to do to identify all of the properties that need to be moved into a reactive- or servlet-specific namespace. I think it's best if we do that before trying to make any good changes and I've labelled the issue as pending design work to reflect this. Thanks anyway.

[polarbear567]

Sure, I'd like to do something after you finish the design.