Add authentication support to URL catalog connector

[ptitzler]

This PR adds

• authentication support for the URL component catalog connector:
• closes Add basic authentication support to URL connector #2793
• support for UI placeholder values for list-valued properties

What changes were proposed in this pull request?

The URL catalog connector now accepts optional credentials and displays a placeholder URL.

• If complete credentials (user id and password/API key) are provided, they are used to fetch every specified URL.
• To use different credentials for different URLs, a user has to configure multiple URL connectors.

The URL connectors' log messages now indicate the name of the connector that is encountering the problem, i.e. if this connector is misconfigured

this error message might appear in the log:

Error. URL catalog connector 'count rows' is not configured properly. Authentication requires a user id and password or API key.

Existing URL component catalog connectors should continue to work as is, without the need for migration.

How was this pull request tested?

• reviewed the output of make docs
• tested the following scenarios:
  • no user id and password provided (should work for resources that allow for public access)
  • user id and password provided (should work for resources that require authentication)
  • user id provided, but no password (should fail - component is not listed in the VPE palette, error is logged in the JupyterLab console)
  • password provided, but no user id (should fail - component is not listed in the VPE palette, error is logged in the JupyterLab console)

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[elyra-bot]

Thanks for making a pull request to Elyra!

To try out this branch on binder, follow this link:

[ptitzler]

The URL placeholder is looking good now. Thanks Martha!

[kiersten-stokes]

LGTM!

[akchinSTC]

@ajbozarth -approved?