feat(cli): allow to install backends from OCI tar files

[mudler]

Description

This PR allows to install backends locally, for instance:

docker build --build-arg BACKEND=llama-cpp -t llama-cpp-backend -f backend/Dockerfile.llama-cpp .
docker save llama-cpp-backend -o llama-backend.tar
local-ai backends install "ocifile://$PWD/llama-backend.tar"

It also now avoid duplication of commands when installing things locally or not, it will work the same across the codebase (hence why more removals).

Notes for Reviewers

This is needed in order to go ahead with #5790 - we need to build some backends on-the-fly to run CI tests.

Signed commits

• Yes, I signed my commits.

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