Add Python Load Tests Combine Dataflow Batch github action

[Amar3tto]

The following jenkins job has been migrated:

https://ci-beam.apache.org/job/beam_LoadTests_Python_Combine_Dataflow_Batch
https://ci-beam.apache.org/job/beam_LoadTests_Python_Combine_Dataflow_Batch_PR

How has it been tested?

The job has been tested on Self-hosted GKE runners ubuntu 20.04.
The following test cases are passed:

• by adding a comment to the PR

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[damccorm]

What is this 2nd echo doing (here and in other configs)?

[Amar3tto]

It replaces job_name substring with the "name+current date".
In .groovy it was:
def now = new Date().format("MMddHHmmss", TimeZone.getTimeZone('UTC'))
...
job_name : "load-tests-python-dataflow-batch-combine-1-${now}"

[damccorm]

Could you add an explanatory comment to this section explaining what we are doing? It is hard to gather intent from the code

[Amar3tto]

Done

[damccorm]

Thanks!