KAFKA-20158: Parameterize session store unit tests to run with headers-aware store format

[bbejeck]

Add headers-enabled variants to SessionWindowedKStreamImplTest and
SessionWindowedCogroupedKStreamImplTest to verify session windowed
aggregations work correctly with the DSL_STORE_FORMAT_HEADERS config.

• Convert aggregate, reduce, count, and materialize tests in
  SessionWindowedKStreamImplTest from @EnumSource to @MethodSource
  to test all combinations of emit strategy and headers mode

• Convert aggregate tests in SessionWindowedCogroupedKStreamImplTest
  from @Test to @ParameterizedTest with @ValueSource(booleans)
  for headers toggling