This commit adds a new `spring.netty.leak-detection` configuration
property that selects the level of memory leak detection for the Netty
engine.

This configuration is applied statically to Netty; this means all
(non-shaded) Netty usages as client or server will be impacted by this
change.

Developers might use this property during development or tests to find
causes of memory leaks when dealing with Netty buffers.

Closes gh-14338