Reorganize server observability packages

Prior to this commit, the server observability support would create a
cycle in Java packages.

This commit refactors the current arrangement to solve this by:

* "flattening" the reactive HTTP instrumentation; this removes the
  dependency to the `ServerWebExchange` and `PathPattern` types
* moving the `observation` package under
  `org.springframework.http.server` and
  `org.springframework.http.server.reactive`

See gh-29477

spring-webmvc/src/test/java/org/springframework/web/servlet/function/support/RouterFunctionMappingTests.java

@@ -26,7 +26,7 @@ import org.junit.jupiter.params.provider.ValueSource;
 
 import org.springframework.context.annotation.AnnotationConfigApplicationContext;
 import org.springframework.http.converter.HttpMessageConverter;
-import org.springframework.http.observation.ServerRequestObservationContext;
+import org.springframework.http.server.observation.ServerRequestObservationContext;
 import org.springframework.web.filter.ServerHttpObservationFilter;
 import org.springframework.web.servlet.HandlerExecutionChain;
 import org.springframework.web.servlet.HandlerMapping;

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/RequestMappingInfoHandlerMappingTests.java

@@ -33,8 +33,8 @@ import org.springframework.core.annotation.AnnotationUtils;
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpMethod;
 import org.springframework.http.MediaType;
-import org.springframework.http.observation.ServerRequestObservationContext;
 import org.springframework.http.server.RequestPath;
+import org.springframework.http.server.observation.ServerRequestObservationContext;
 import org.springframework.lang.Nullable;
 import org.springframework.stereotype.Controller;
 import org.springframework.util.MultiValueMap;