Beam python nexmark benchmark suite

Nexmark is a suite of queries (pipelines) used to measure performance and non-regression in Beam. Currently it exists in java sdk: https://github.com/apache/beam/tree/master/sdks/java/testing/nexmark/src/main/java/org/apache/beam/sdk/nexmark

In this project we would like to create the nexmark benchmark suite in python sdk equivalent to what BEAM has for java. This allows us to determine performance impact on pull requests for python pipelines.