feat: make SQLAlchemy pool class configurable + parse JSON in engine settings to dict

[thinkh]

Related issue: https://github.com/Caleydo/tdp_bi_bioinfodb/issues/1439

Related PR datavisyn/tdp_core#855

Developer Checklist (Definition of Done)

Issue

• All acceptance criteria from the issue are met
• Tested in latest Chrome/Firefox

UI/UX/Vis

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Code

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PR

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Summary of changes

• make SQLAlchemy pool class configurable by passing a valid SQL Alchemy pool to poolclass
• parse JSON in engine settings to dict

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Thanks for creating this pull request 🤗