`(validate)` field option should generate code for calling `validate()` for nested messages

[alexander-yevsyukov]

This does not happen in new Validation. For example, see io.spine.server.stand.EntityQueryProcessorTest.failOnInvalidQuery().

The test now passed because EntityQueryProcessor.process(Query) was patched to manually check()s instances of Query (which is a good idea anyway) AND query.getTarget() (which should be done by Query.validate()).

[alexander-yevsyukov]

This needs investigation. It's probably fixed already, but needs confirmation and coverage by tests.

[alexander-yevsyukov]

Confirmed in this PR: #298.