Add support for 'export' command to elyra-pipeline CLI

[ptitzler]

Adds support for the elyra-pipeline export command, which allows for export of a pipeline to a runtime-native format (KFP and AA). The command mimics the functionality that the VPE already provides).

$ elyra-pipeline --help
Usage: elyra-pipeline [OPTIONS] COMMAND [ARGS]...

  Run Elyra pipelines in your local environment or submit them to an
  external service, such as Kubeflow Pipelines or Apache Airflow.

  Find more information at: https://elyra.readthedocs.io/en/latest/

Options:
  --version  Show the version and exit.
  --help     Show this message and exit.

Commands:
  describe  Display pipeline summary
  export    Export a pipeline to a runtime-specific format
  run       Run a pipeline in your local environment
  submit    Submit a pipeline to be executed on the server
  validate  Validate pipeline

$ elyra-pipeline export --help
Usage: elyra-pipeline export [OPTIONS] PIPELINE_PATH

  Export a pipeline to a runtime-specific format

Options:
  --runtime-config TEXT  Runtime configuration name.  [required]
  --output PATH          Exported file name (including optional path).
                         Defaults to  the current directory and the pipeline
                         name.

  --overwrite            Overwrite output file if it already exists.
  --help                 Show this message and exit.

output:

• if not specified, the output file is named after the pipeline file, with the appropriate extension added. and stored in the current directory. Example: elyra-pipeline export analysis.pipeline ... --> ./analysis.yaml (KFP) ./analysis.py (AA)
• if the specified value ends in yaml or .yml (KFP) .py (AA) it is used as output name, as is:
  • directories are created, if necessary
  • elyra-pipeline export analysis.pipeline --output /path/to/file.yaml ... --> output: /path/to/file.yaml
  • elyra-pipeline export analysis.pipeline --output /path/to/file.py ... --> output: /path/to/file.py
• if the specified value does not end in yaml or .yml (KFP) .py (AA), it is interpreted as a directory name and the exported file named after the pipeline file, with the appropriate extension added
  • elyra-pipeline export analysis.pipeline --output /path/to/ ... --> output: /path/to/analysis.yaml

overwrite:

• if specified, the output file is overwritten if it already exists

Documentation updates:

• new export subsection
• (unrelated) moved the describe content to its own subsection

Closes #2571

What changes were proposed in this pull request?

• Add support for export command to elyra-pipeline CLI
• Update https://elyra.readthedocs.io/en/latest/user_guide/pipelines.html
• Update https://elyra.readthedocs.io/en/latest/user_guide/command-line-interface.html
• Add unit tests to elyra/tests/cli/test_pipeline_app.py
• Update existing unit tests (moved assets to resources, simplified existing tests, use pathlib instead of os., etc) in elyra/tests/cli/test_pipeline_app.py
• Tweak implementation in elyra/cli/pipeline_app.py

How was this pull request tested?

• Manual testing using elyra-pipeline CLI
• Added unit test cases to elyra/tests/cli/test_pipeline_app.py
• Reviewed the output of make docs

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[ptitzler]

Remaining todo's:

• Documentation updates
• Improve elyra/cli/pipeline_app.py

[kevin-bates]

These changes look good @ptitzler and there's some nice clean-up. I just had a couple of comments and have not taken it for a spin yet either.