Add support for metadata import in the CLI

[sandhya-nayak]

What changes were proposed in this pull request?

Fixes #2500

Add support for metadata import in the cli
Add tests for import subcommand
Update documentation

How was this pull request tested?

Ran make clean, make install, make test, make docs to verify that the installation and tests were successful and documentation updates were rendered correctly.

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[kevin-bates]

Thank you @sandhya-nayak - I added a few initial comments but this looks good overall.

[kevin-bates]

This looks really good @sandhya-nayak - thank you. Just had a couple more minor comments.

[kevin-bates]

Excellent stuff @sandhya-nayak - thank you!

[ptitzler]

I've completed my review/testing. If you make the suggested changes above to keep the code style consistent this PR should be good to go.

[ptitzler]

Could you also update the heading in https://elyra.readthedocs.io/en/latest/user_guide/command-line-interface.html#list-create-update-export-and-delete-metadata to include 'import'

[ptitzler]

Very nice. LGTM! Thank you for helping us make our CLI story more compelling.

[akchinSTC]

Very nice. LGTM! Thank you for helping us make our CLI story more compelling.

+1
This is awesome sandhya!