Simplify input of Kubernetes secrets in the pipeline editor

[ptitzler]

Is your feature request related to a problem? Please describe.
Version 3.9.0 introduced support for Kubernetes secrets, enabling users to make sensitive information available to notebooks/scripts via environment variables, without exposing sensitive information in the pipeline editor^, the pipeline file^^, or the runtime environment^^^. The current input format ENV_VAR=secret-name:secret-key (see instructions) is not user friendly.

^ the secret's value is never displayed in the GUI, nor is it ever accessed or logged by Elyra
^^ only the secret name and the secret's key is stored in the .pipeline file and generated DAGs, but not its value
^^^ assuming that access to the Kubernetes secret is protected and its value not exposed by the notebooks/scripts during execution (e.g. via logging)

Describe the solution you'd like

Replace the current string based input with a widget that's intuitive to use.

See related issue #2738, which raises the same requirement for data volume input.