Add reuse mechanism for OpenSergoClient

[jnan806]

Issue Description

Type: feature request

Describe what feature you want

In current version, OpenSergoClient uses prototype mode.
SDK will create a new instance when invoke NewOpenSergoClient, even if the endpoint is the same.
It is very expensive without reuse mechanism, so we need a reuse mechanism to avoid it.

[mayooot]

@jnan806 Hi, I use a map with host:port as key and openSergoClient as value to control reuse, and use sync.map to avoid concurrent access problems.

I think it should work.