Add support for pluggable explainer runtimes

[TimKleinloog]

What this PR does / why we need it:
For additional context see #3398

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Implements #3398
New clean PR resulting from #3418

Type of changes

• New feature (non-breaking change which adds functionality)

Feature/Issue validation/testing:

Please describe the tests that you ran to verify your changes and relevant result summary. Provide instructions so it can be reproduced.
Please also list any relevant details for your test configuration.

• Run make deploy-dev and deploy the new explainer sample that uses an explainer runtime docs/samples/v1beta1/explanation/deeploy-shap/deeploy-sample.yaml

Special notes for your reviewer:

1. Some changes include naming decisions: ModelSpec is reused for ExplainerSpec, how to both use PredictorExtensionSpec and ExplainerExtensionSpec in ModelSpec?

Checklist:

• Have you added unit/e2e tests that prove your fix is effective or that this feature works?
• Has code been commented, particularly in hard-to-understand areas?
• Have you made corresponding changes to the documentation?

Release note:

You can now configure a model in the ExplainerSpec to plug any arbitrary framework

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[TimKleinloog]

@terrytangyuan I started this clean PR here after #3443 was merged. The tests look to succeed now, is it already possible to merge this?

[terrytangyuan]

This is currently defined in predictor.go. Perhaps we should extract that out?

[TimKleinloog]

This is actually defined in model.go which I renamed (was predictor_model.go) so it is more logical that is used in both explainer.go and predictor.go. In model.go the PredictorExtensionSpec is used which is defined in predictor.go do you think we should extract that out?

[terrytangyuan]

@TimKleinloog Can you limit the scope of this PR to only include the new feature? New runtimes can be added via a separate PR as an example for now.

[TimKleinloog]

@TimKleinloog Can you limit the scope of this PR to only include the new feature? New runtimes can be added via a separate PR as an example for now.

I will open a new PR and remove the explainer runtime example from the scope of this PR 👌

[yuzisun]

@TimKleinloog Can you help resolve the conflicts?

[TimKleinloog]

@TimKleinloog Can you help resolve the conflicts?

Done, hope this will be one of the last conflicts to resolve :)