Allow /health to be accessed anonymously irresepctive of its sensitivity

The changes in 3bb598a overload the health endpoint's sensitive
property such that it's now considered sensitive if management
security is enabled. When an endpoint is sensitive anonymous
access is prevented. This breaks the health endpoint which should
return a filtered view of the server's health when it's accessed
anonymously rather than rejecting the request.

This commit introduces AnonymouslyAccessibleMvcEndpoint, a marker
extension of the MvcEndpoint interface. It is implemented by
HealthMvcEndpoint. ManagementSecurityAutoConfiguration has been
updated to allow anonymous access to endpoints that aren't sensitive
or that implement AnonymouslyAccessibleMvcEndpoint.

Fixes gh-2015