Pipeline editor: Allow for configuration of labels

[ptitzler]

Extend the pipeline editor to allow for adding of kubernetes labels in generic and custom components. A pipeline default can be configured in the pipeline properties. Support is implemented for Kubeflow Pipelines and Apache Airflow.

The key/value pair can be used to query Kubernetes resources:

$ kubectl get pods -n kubeflow-user-example-com -l project=demo
NAME                     READY   STATUS      RESTARTS   AGE
lambda-vh2zp-982439046   0/2     Completed   0          38s

The pipelines documentation describes the new property:

Closes #2941

What changes were proposed in this pull request?

• See above

How was this pull request tested?

• reviewed the output of make docs
• Added validation tests (for valid and invalid inputs)
• Updated handlers test
• Updated integration tests

pipeline defaults:

• no label key specified
• no label value specified
• invalid label key specified
• valid key/value

generic nodes (kfp):

• no label key specified
• no label value specified
• invalid label key specified
• valid key/value

custom nodes (kfp):

• no label key specified
• no label value specified
• invalid label key specified
• valid key/value

generic nodes (airflow):

• no label key specified
• no label value specified
• invalid label key specified
• valid key/value

custom nodes (airflow):

• no label key specified
• no label value specified
• invalid label key specified
• valid key/value

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[elyra-bot]

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[kiersten-stokes]

Cool! LGTM