"beam:java:boundedsource" not supported with python optimizer

python pipeline optimizer does not handle external transforms.

Relevant error stack

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ERROR: test_external_transforms (__main__.FlinkRunnerTestOptimized)
\--\--\--\--\--\--\--\--\--\--\--\--\--\--\--\--\--\--\--\--\--\--\--\--\--\--\--\--\--\--\--\--\--\--\--
Traceback (most recent call last):
File "/tmp/beam/beam/sdks/python/apache_beam/runners/portability/flink_runner_test.py", line 174, in test_external_transforms
assert_that(res, equal_to([i for i in range(1, 10)]))
File "/tmp/beam/beam/sdks/python/apache_beam/pipeline.py", line 426, in __exit__
self.run().wait_until_finish()
File "/tmp/beam/beam/sdks/python/apache_beam/runners/portability/portable_runner.py", line 436, in wait_until_finish
self._job_id, self._state, self._last_error_message()))
RuntimeError: Pipeline test_external_transforms_1557358286.71_f49d7fd6-7c14-4ded-8946-3ac3dad4d4c9 failed in state FAILED: java.lang.RuntimeException: Error received from SDK harness for instruction 4: Traceback (most recent call last):
File "/tmp/beam/beam/sdks/python/apache_beam/runners/worker/sdk_worker.py", line 157, in _execute
response = task()
File "/tmp/beam/beam/sdks/python/apache_beam/runners/worker/sdk_worker.py", line 190, in
self._execute(lambda: worker.do_instruction(work), work)
File "/tmp/beam/beam/sdks/python/apache_beam/runners/worker/sdk_worker.py", line 333, in do_instruction
request.instruction_id)
File "/tmp/beam/beam/sdks/python/apache_beam/runners/worker/sdk_worker.py", line 353, in process_bundle
instruction_id, request.process_bundle_descriptor_reference)
File "/tmp/beam/beam/sdks/python/apache_beam/runners/worker/sdk_worker.py", line 305, in get
self.data_channel_factory)
File "/tmp/beam/beam/sdks/python/apache_beam/runners/worker/bundle_processor.py", line 501, in __init__
self.ops = self.create_execution_tree(self.process_bundle_descriptor)
File "/tmp/beam/beam/sdks/python/apache_beam/runners/worker/bundle_processor.py", line 545, in create_execution_tree
descriptor.transforms, key=topological_height, reverse=True)])
File "/tmp/beam/beam/sdks/python/apache_beam/runners/worker/bundle_processor.py", line 467, in wrapper
result = cache[args] = func(*args)
File "/tmp/beam/beam/sdks/python/apache_beam/runners/worker/bundle_processor.py", line 528, in get_operation
in descriptor.transforms[transform_id].outputs.items()
File "/tmp/beam/beam/sdks/python/apache_beam/runners/worker/bundle_processor.py", line 527, in
for tag, pcoll_id
File "/tmp/beam/beam/sdks/python/apache_beam/runners/worker/bundle_processor.py", line 467, in wrapper
result = cache[args] = func(*args)
File "/tmp/beam/beam/sdks/python/apache_beam/runners/worker/bundle_processor.py", line 531, in get_operation
transform_id, transform_consumers)
File "/tmp/beam/beam/sdks/python/apache_beam/runners/worker/bundle_processor.py", line 790, in create_operation
return creator(self, transform_id, transform_proto, payload, consumers)
File "/tmp/beam/beam/sdks/python/apache_beam/runners/worker/bundle_processor.py", line 957, in create
parameter.source, factory.context),
File "/tmp/beam/beam/sdks/python/apache_beam/utils/urns.py", line 113, in from_runner_api
parameter_type, constructor = cls._known_urns[fn_proto.spec.urn]
KeyError: u'urn:beam:java:boundedsource:v1'

Imported from Jira [BEAM-7252](https://issues.apache.org/jira/browse/BEAM-7252). Original Jira may contain additional context.
Reported by: angoenka.