feat: gallery repository

[mudler]

Description

This PR is multi fold:

• introduce gallery repositories that can be defined via environment variable (for now)
• automatically detect and uncompress .tar/.tar.gz/etc archives if downloaded by the gallery ( fixes feat: gallery should support automatic extractions #651 )
• install models from repositories
• list available models
• provide compatibility support with the old /v1/engines/:model/completions endpoint
• detect installed models (this is done as a first pass by looking if the model has a configuration file existing in the models folder)

TODO:

• detect installed models during listing
• better state for installed models (where are coming from? where to keep system information?)
• docs

Notes for Reviewers

Signed commits

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