[OpenSTEF 4.0] Adding shap explainability support for forecasters

[egordm]

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🌍 Background

Important

Changes should be done by forking OpenSTEF 4.0.0 tracking branch (release/v4.0.0) and then make a PR back onto release/v4.0.0 branch.
Please read the contributing guide before you start.

❗Priority (What if we don't do this?/Are there any deadlines? etc.)

Definition of Done:

✅ Acceptance criteria

• Check with SIA branch so no double work is done (OpenSTEF Meta V0.1 #771)

📄 Documentation criteria:

Interfaces with other products and services

Update relevant confluence pages

🧪 Test criteria:

Unit tests written and passed

⌛ Dependencies:

N/A

🚀 Releasing:

N/A

Other information:

[egordm]

This one is part of the meta package. Once that is merged, it will be available
#771