ability to perform vector search via gRPC service

[gsantopaolo]

We need the ability to switch between the actual search on the vector database build in the Go code to a search performed via a gRPC service.
This way, we will be able to perform Data Science ops on the vector search.

Env var on cognix-api:
API-VECTOR-SEARCH:

Allowed values:
INTERNAL
GRPC-SERVICE

Default (if not provided or not able to find in env vars, INTERNAL)

INTERNAL: the API performs a vector search inside the go code, the search we have right now.

GRPC-SERIVCE: The API call a gRPC unary service described below that will return a list of document IDs

With this list the API will complete the search flow invoking the LLM

gRPC search service definition

`// gRPC Service
syntax = "proto3";

package com.cognix;
option go_package = "backend/core/proto;proto";

message SearchRequest {
string content = 1;
string model = 2;
}

message SearchResponse {
repeated int64 vector = 1;
}

service SearchService {
rpc VectorSearch (SearchRequest) returns (SearchResponse) {}
}
`