Add actuator specific ObjectMapper

Prior to this commit, Actuator endpoints would use the application
ObjectMapper instance for serializing payloads as JSON. This was
problematic in several cases:

* application-specific configuration would change the actuator endpoint
output.
* choosing a different JSON mapper implementation in the application
would break completely some endpoints.

Spring Boot Actuator already has a hard dependency on Jackson, and this
commit uses that fact to configure a shared `ObjectMapper` instance that
will be used by the Actuator infrastructure consistently, without
polluting the application context.

This `ObjectMapper` is used in Actuator for:

* JMX endpoints
* Spring MVC endpoints with an HTTP message converter
* Spring WebFlux endpoints with an `Encoder`
* Jersey endpoints with a `ContextResolver<ObjectMapper>`

For all web endpoints, this configuration is limited to the
actuator-specific media types such as
`"application/vnd.spring-boot.actuator.v3+json"`.

Fixes gh-12951

src/checkstyle/import-control.xml

@@ -46,6 +46,9 @@
 				<disallow pkg="org.springframework.web.servlet" />
 			</subpackage>
 		</subpackage>
+		<subpackage name="json">
+			<allow pkg="org.springframework.http.converter" />
+		</subpackage>
 	</subpackage>
 
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