feat(loader): enhance single active backend to support LRU eviction (#7535)

* feat(loader): refactor single active backend support to LRU

This changeset introduces LRU management of loaded backends. Users can
set now a maximum number of models to be loaded concurrently, and, when
setting LocalAI in single active backend mode we set LRU to 1 for
backward compatibility.

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

* chore: add tests

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

* Update docs

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

* Fixups

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

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Signed-off-by: Ettore Di Giacinto <mudler@localai.io>