This commit simplifies the Jackson-related auto-configuration that’s
applied when Spring HATEOAS and Spring Data REST are on the classpath.

Previously, Boot used Jackson2HalModule to apply the HAL-related
ObjectMapper configuration to the context’s primary ObjectMapper. This
was to allow HAL-formatted responses to be sent for requests accepted
application/json (see gh-2147). This had the unwanted side-effect of
polluting the primary ObjectMapper with HAL-specific functionality.
Furthermore, Jackson2HalModule is an internal of Spring HATEOAS that
@olivergierke has asked us to avoid using.

This commit replaces the use of Jackson2HalModule with a new approach.
Now, the message converters of any RequestMappingHandlerAdapter beans
are examined and any TypeConstrainedMappingJackson2HttpMessageConverter
instances are modified to support application/json in addition to their
default support for application/hal+json. This behaviour can be disabled
by setting spring.hateoas.use-hal-as-default-json-media-type to false.
This property is named after Spring Data REST’s configuration option
which has the same effect when using Spring Data REST. The new property
replaces the old spring.hateoas.apply-to-primary-object-mapper property.

Previously, when Spring Data REST was on the classpath,
JacksonAutoConfiguration would be switched off resulting in the context
containing multiple ObjectMappers, none of which was primary.

This commit configures RepositoryRestMvcAutoConfiguration to run after
JacksonAutoConfiguration. This gives the latter a chance to create its
primary ObjectMapper before the former adds its ObjectMapper beans to
the context.

Previously, the actuator’s hypermedia support assumed that the
HttpMessageConverters bean would contain every HttpMessageConverter
being used by Spring MVC. When Spring HATEOAS is on the classpath this
isn’t the case as it post-processes RequestMappingHandlerAdapter beans
and adds a TypeConstrainedMappingJackson2HttpMessageConverter to them.
This wasn’t a problem in the past as the primary ObjectMapper, used by a
vanilla MappingJackson2HttpMessageConverter, was configured with Spring
HATEOAS’sJackson2HalModule. Now that this pollution has been tidied up
the assumption described above no longer holds true. MvcEndpointAdvice,
which adds links to the actuator’s json responses, has been updated
to look at the HttpMessageConverters of every
RequestMappingHandlerAdapter when it’s trying to find a converter to
use to write a response with additional hypermedia links.

Integration tests have been added to spring-boot-actuator to ensure
that the changes described above have not regressed the ability to
configure its json output using spring.jackson.* properties (see
gh-1729).

Closes gh-3891