feat: migrate to echo

[mudler]

Description

This PR migrates to echo. One of the main reason is the lack of support of detecting properly client cancellation https://github.com/valyala/fasthttp/issues/468#issuecomment-439665014, which would have the unwanted effect of booking resources for AI inferencing even if clients disconnect.

I've tried several workarounds in https://github.com/mudler/LocalAI/pull/7187 , but none of these was reliable ( had to revert in https://github.com/mudler/LocalAI/pull/7266 ). Finally, I've came to the conclusion that the effort involved in maintaining go-fiber (which is based over fasthttp) is higher overall for the little gain that we would have (the real computation is offloaded, so we don't perform high-intensive tasks in the golang layer) by using it.

Notes for Reviewers

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