Add Base Retrieval Action and Use Pydantic

[20001LastOrder]

Description

This pr closes #361 by implementing the base retrieval action. It also adapts Pydantic to actions as mentioned in #372.

Note

It potentially breaks existing custom actions since now the actions have to be defined following the Pydantic schema. I updated the corresponding tutorial for PDF reader and citation validator

It does not break other things, including the slack bot

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Maintenance
• New release

Related issues

Mention related GitHub and Linear issues. E.g. Closes #xxx or Fixes #xxx. Otherwise delete this section.

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• Changed code is self-explanatory and/or I added comments
• I updated the documentation (docstrings, /docs)
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