Add support for Jan as an LLM provider

[simanadler]

Description

Adds Jan as an LLM provider. See issue #3724 for description and motivation.

Fixes #3724

Type of change

Please delete options that are not relevant.

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

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

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• Unit Test
• Quickstart example

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

Maintainer Checklist

• closes Add Support for Jan as an LLM Provider #3724
• Made sure Checks passed

[CLAassistant]

All committers have signed the CLA.

[sima5585]

New here but excited about contributing to this amazing project!
Any idea when this PR will be reviewed @parshvadaftari ?