The contains syntax was added in #30874 but the skips were not properly
put in place.
The java runner has the feature so the tests will run as part of the
build, but language clients will be able to support it at their own
pace.
Suggestion responses were previously serialized as streamables which
made writing suggesters in plugins with custom suggestion response types
impossible. This commit makes them serialized as named writeables and
provides a facility for registering a reader for suggestion responses
when registering a suggester.
This also makes Suggestion responses abstract, requiring a suggester
implementation to provide its own types. Suggesters which do not need
anything additional to what is defined in Suggest.Suggestion should
provide a minimal subclass.
The existing plugin suggester integration tests are removed and
replaced with an equivalent implementation as an example
plugin.