Designate kfp python package as optional dependency

[akchinSTC]

Closes #3139

What changes were proposed in this pull request?

How was this pull request tested?

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[shalberd]

might remove the newline here, small nitpicking by me

[shalberd]

Testing this, the changes from the PR, out. Currently, still getting, due to kfp not present (no module named kfp).

I did make package-ui, make build-dependencies and make build-server and then installed the wheel file with the option [gitlab] instead of [all] or [kfp-tekton] as I am only using airflow with Elyra. It is working in the sense that I now no longer have kfp in pip list ...

https://elyra.readthedocs.io/en/stable/getting_started/installation.html#packaging

So now I have a python environment with no kfp python package anymore, so far, so good. Except:

0 00:24:08.592 ServerApp] Error loading schemas for SchemasProvider 'code-snippets' - No module named 'kfp'
[E 2024-04-30 00:24:08.593 ServerApp] Error loading schemas for SchemasProvider 'component-catalogs' - No module named 'kfp'
[E 2024-04-30 00:24:08.594 ServerApp] Error loading schemas for SchemasProvider 'runtimes' - No module named 'kfp'
[E 2024-04-30 00:24:08.595 ServerApp] Error loading schemas for SchemasProvider 'runtimes-images' - No module named 'kfp'
[W 2024-04-30 00:24:08.595 ServerApp] The following schemaspaces have no schemas: ['code-snippets', 'runtimes', 'runtime-images']

https://github.com/elyra-ai/elyra/blob/main/elyra/metadata/schema.py#L260

There would probably need to be a way to remove the hard-coded kfp runtime from RuntimeProcessorType enum https://github.com/kevin-bates/elyra/blob/main/elyra/pipeline/runtime_type.py#L25

Cause there is a difference between active runtimes from the config we all know and installed runtimes that then make a problem once kfp is not installed anymore.

I get my hunch from @kevin-bates https://github.com/elyra-ai/elyra/pull/2263/files#diff-2cb56468c742ac4f0ea118665f54e0424289b6cf4fd47233342eebd08034a889

@harshad16 what do you think?