Prior to this commit, `HealthContributor` would be exposed under the
main `HealthEndpoint` and subgroups, `HealthEndpointGroups`. Groups are
driven by configuration properties and there was no way to contribute
programmatically new groups.
This commit introduces the `HealthEndpointGroupsRegistry` (a mutable
version of `HealthEndpointGroups`) and a
`HealthEndpointGroupsRegistryCustomizer`. This allows configurations to
add/remove groups during Actuator auto-configuration.
Closes gh-20554
b680db6cd8