Improve JSON output of 'elyra-pipeline describe' command

[ptitzler]

Improve the JSON-formatted output of the elyra-pipeline describe command. Previously custom component dependency metadata was reported as a string, which made it unnecessarily difficult for users to consume the information programatically.

Example:

$ elyra-pipeline describe --json my-airflow.pipeline

Old output:

   ...
   "dependencies": {
        "scripts": [],
        "notebooks": [],
        "files": [],
        "custom_components": [
            "{\"catalog_type\": \"airflow-package-catalog\", \"component_ref\": {\"airflow_package\": \"apache_airflow-1.10.15-py2.py3-none-any.whl\", \"file\": \"airflow/operators/bash_operator.py\"}}"
        ],
        "container_images": [],
        ...

New output:

    ...
    "dependencies": {
        "scripts": [],
        "notebooks": [],
        "files": [],
        "custom_components": [
            {
                "catalog_type": "airflow-package-catalog",
                "component_ref": {
                    "airflow_package": "apache_airflow-1.10.15-py2.py3-none-any.whl",
                    "file": "airflow/operators/bash_operator.py"
                }
            }
        ],
        "container_images": [],
        ...

Closes #2784

What changes were proposed in this pull request?

• Render custom_components metadata as dictionary instead of string
• Add new test case
• Fix incompatible test pipeline

How was this pull request tested?

• New test case
• Manual testing using pipelines that utilize zero, one or more custom components.

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[elyra-bot]

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[kiersten-stokes]

This looks so much better!

[ptitzler]

This looks so much better!

... says the JSON parser 😉