Previously, actuator endpoints were registered with Jersey upon
injection of the ResourceConfig bean into a registrar class rather than
using a ResourceConfigCustomizer. This was done to fix a problem
when running the Actuator on a separate port where the main application
context's customizers were also applied to the management context,
breaking the singleton contract for those resources. This approach
meant that the registration could be performed at any point after the
ResourceConfig had been created. When Jersey's configured as a Filter
this resulted in the registration failing as the attempt was being made
after the Filter lifecyle callbacks which make the ResourceConfig
immutable.

This commit reworks the endpoint registration to be performed using a
ManagementContextResourceConfigCustomizer, a resource config customizer
that's only applied to the ResourceConfig that's used by the Actuator.
When there's a separate management context, this ResourceConfig is
created by the Actuator's auto-configuration and the management context
resource config customizers are applied to it during its creation. The
main application's customizers are not applied. When the actuator is
using the same context as the main application, this ResourceConfig is
created by the main application. In this case a
ResourceConfigCustomizer is defined that delegates to all
ManagementContextResourceConfigCustomizers, allowing them to register
the actuator endpoints with the main ResourceConfig.

Fixes gh-25262