Develop a Python cross-language wrapper for Java BigTableIO

Java connector: [https://github.com/apache/beam/blob/master/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/bigtable/BigtableIO.java](https://github.com/apache/beam/blob/master/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/bigtable/BigtableIO.java)

Documentation for developing Python cross-language transforms:

[https://beam.apache.org/documentation/programming-guide/#1311-creating-cross-language-java-transforms](https://beam.apache.org/documentation/programming-guide/#1311-creating-cross-language-java-transforms)

[https://beam.apache.org/documentation/sdks/python-multi-language-pipelines/](https://beam.apache.org/documentation/sdks/python-multi-language-pipelines/)

Existing cross-language transform for Cloud Spanner might serve as a good example to follow.

[https://github.com/apache/beam/blob/master/sdks/python/apache_beam/io/gcp/spanner.py](https://github.com/apache/beam/blob/master/sdks/python/apache_beam/io/gcp/spanner.py)

[https://github.com/apache/beam/blob/a4e112974d8f3f51e646d9e714a4f8d456d69b4d/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/spanner/SpannerTransformRegistrar.java](https://github.com/apache/beam/blob/a4e112974d8f3f51e646d9e714a4f8d456d69b4d/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/spanner/SpannerTransformRegistrar.java)

Imported from Jira [BEAM-13607](https://issues.apache.org/jira/browse/BEAM-13607). Original Jira may contain additional context.
Reported by: chamikara.