Prior to this commit, every EndpointMBean used its own ObjectMapper.
Each of these ObjectMappers was created using new ObjectMapper() with
no opportunity for configuration.

This commit uses the ObjectMapper from the application context and
shares it among all EndpointMBeans. This gives the user control over
the ObjectMapper’s configuration using spring.jackson.* properties,
their own Jackson2ObjectMapperBuilder bean, etc. In the absence of an
ObjectMapper in the application context a single ObjectMapper is
instantiated and is used by all EndpointMBeans instead.

To allow the ObjectMapper to be shared, a number of constructors have
been overloaded to also take the ObjectMapper as a parameter. In these
cases the old constructor has been preserved for backwards compatibility
but has been deprecated.

Closes gh-2393