Ignore flushes on ServletServerHttpResponse output stream

Prior to this commit, flush calls on the output stream returned by
`ServletServerHttpResponse#getBody` would be delegated to the Servlet
response output stream.
This can cause performance issues when `HttpMessageConverter` and other
web components write and flush multiple times to the response body.
Here, the Servlet container is in a better position to flush to the
network at the optimal time and buffer the response body until then.

This is particularly true for `HttpMessageConverters` when they flush
many times the output stream, sometimes due to the underlying codec
library. Instead of revisiting the entire message converter contract, we
are here ignoring flush calls to that output stream.

This change does not affect the client side, nor the
`ServletServerHttpResponse#flush` calls.

This commit also introduces a new Spring property
`"spring.http.response.flush.enabled"` that reverts this behavior change
if necessary.

Closes gh-36385

framework-docs/modules/ROOT/pages/appendix.adoc

@@ -81,6 +81,11 @@ resolvable otherwise. See
 {spring-framework-api}++/core/env/AbstractEnvironment.html#IGNORE_GETENV_PROPERTY_NAME++[`AbstractEnvironment`]
 for details.
 
+| `spring.http.response.flush.enabled`
+| Configures the Spring MVC `ServletServerHttpResponse` to allow flushing on the `OutputStream`
+returned by `ServletServerHttpResponse#getBody()`. By default, such flush calls are ignored and
+only `ServletServerHttpResponse#flush()` will actually flush the response to the network.
+
 | `spring.jdbc.getParameterType.ignore`
 | Instructs Spring to ignore `java.sql.ParameterMetaData.getParameterType` completely.
 See the note in xref:data-access/jdbc/advanced.adoc#jdbc-batch-list[Batch Operations with a List of Objects].