feat(grpc): backend SPI pluggable in embedding mode

[coyzeng]

Description

Make Backend SPI pluggable in embedding mode. Like:

package grpc

import "github.com/go-skynet/LocalAI/pkg/grpc"

type LLM grpc.LLM

func Provide(addr string, llm LLM) {
	grpc.Provide(addr, llm)
}

func main() {
       ...
       options.WithExternalBackend("llama2" addr)
       app, err := api.App(options...)
       ...
}

This PR fixes #

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