This moves the feature from a unconditional application listener that
can load gRPC types to an auto-configuration that backs off if the
necessary classes are not present.
Also updated the smoke tests to actually use the feature.
Closes gh-50825
Add a testing module that can be used for testing Spring gRPC
server or client applications.
Closes gh-49046
Co-authored-by: Phillip Webb <phil.webb@broadcom.com>