…onTestHarness

This commit introduces an initial implementation of a test harness for PTFs, for use
in unit tests that do not require a running Flink cluster.

The implementation supports setting up the harness by configuration various test
parameters, like fixtures for scalar arguments, datatypes for table arguments and
partition settings for table arguments with set semantics.

The harness on build does type and structure validation, as well as ensuring the
test setup can handle the arguments defined on the PTF.

It supports PTFs that use scalar, set semantic table and row semantic table arguments,
as well as PTFs that have multiple of each. It supports PASS_COLUMN_THROUGH and
OPTIONAL_PARTITION_BY traits.

It currently does not support State, or Context (so no timers). It also does not enforce
some static argument traits like SUPPORTS_UPDATES, REQUIRE_UPDATE_BEFORE.