[AIRFLOW-5711] Add fallback for connection's project ID in Dataflow integration

[mik-laj]

This PR is one of a series that aims to improve this integration
https://issues.apache.org/jira/browse/AIRFLOW-5697

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[codecov-io]

Codecov Report

Merging #6383 into master will increase coverage by 3.37%.
The diff coverage is 95.23%.

@@            Coverage Diff             @@
##           master    #6383      +/-   ##
==========================================
+ Coverage   80.59%   83.97%   +3.37%     
==========================================
  Files         626      627       +1     
  Lines       36243    36541     +298     
==========================================
+ Hits        29211    30684    +1473     
+ Misses       7032     5857    -1175

Impacted Files	Coverage Δ
airflow/gcp/operators/dataflow.py	99.07% <75%> (ø)	⬆️
airflow/gcp/hooks/dataflow.py	92.24% <97.36%> (+1.08%)	⬆️
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	45.25% <0%> (-46.72%)	⬇️
airflow/kubernetes/kube_client.py	33.33% <0%> (-41.67%)	⬇️
...rflow/contrib/operators/kubernetes_pod_operator.py	70.14% <0%> (-28.36%)	⬇️
airflow/models/dagbag.py	85.24% <0%> (-7.18%)	⬇️
airflow/example_dags/example_trigger_target_dag.py	90% <0%> (-2.31%)	⬇️
airflow/api/common/experimental/delete_dag.py	86.66% <0%> (-0.84%)	⬇️
... and 86 more

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[potiuk]

Love it. Nice and simple