feat(py): refactoring a Plugin abstraction

[timere-nemo]

Description:

• Refactored a Plugin abstraction to make it more versatile in use.
• Updated the VertexAI plugin and a code sample

Problem:

1. Current implementation assumes that only one model is defined, whereas we may have multiple models.
2. It is unclear why _model_callback needs to be defined in the Plugin. Since each model (or model family) has its own execution logic, it should provide its own callback

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

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