Closes gh-2024

Closes gh-23972

Closes gh-23976

Currently, if you have an optional event listener (via a @Bean method returning `null`)
this causes the event multicaster to explode violently.  Now, we just safely skip it.

Closes gh-23958

See gh-23970

Previously, spring-webmvc and spring-webflux both contained tests
that would create gzipped files, write them to the filesystem
alongside the project's compiled test classes, and configure them to
be deleted on JVM exit. The output location placed the files on the
classpath, polluting it for every subsequent test that used the same
ClassLoader. The test-sources plugin combined with Gradle's use of
worker JVMs, broadens the scope of this pollution to other, downstream
projects in the same build. For example, the tests for
spring-websocket will have a different classpath depending on whether
or not the tests for spring-webmvc have already been run on the same
worker as part of the current build.

This commit updates the spring-webmvc and spring-webflux modules to
introduce a new JUnit Jupiter extension, GzipSupport. This extension
allows gzipped files to be created via an injectable GzippedFiles
class and automatically deletes each created file in an after-each
callback. This ensures that a gzipped file only exists on the
classpath for the duration of the test that needs it, avoiding the
pollution of the classpath of any subsequent tests.

Closes gh-23970

Closes gh-23971

Closes gh-23409

Closes gh-23954

Closes gh-23956

See gh-1954

Closes gh-1954

Closes gh-22396

Closes gh-22715

Closes gh-23957

Closes gh-23417

See gh-23417

Closes gh-23955

Apply default Accept header for HttpUrlConnection only.

See gh-23855

Closes gh-23740

This commit notifies the Synchronoss listener that the buffer stream
has ended.

Closes gh-23768

Closes gh-23936

Closes gh-23908

Closes gh-23907