Add OPA policies for NSM registries

[denis-tingaikin]

Motivation

Probably we need to add authorization policies for NSM registries to cover scenarios such as

1. NSE/NSMgr/xconnNSE should be able to unregister only itself. NSE/NSMgr/xconn NSE should not be able to unregister another NSE/NSMgr/xconn NSE.
2. Only NSMgr/vl3 NSE should be able to search NSE in the registry.
3. Only NSMgr and other registries can use Bulk API.

Problem of implementation

We need to somehow pass Path data or other authorization data to registries.

Potential solution

We can use pkg grpc/metadata for passing authorization data for registries.

import "google.golang.org/grpc/metadata"
...  
ctx = metadata.NewContext( 
    ctx, 
    metadata.Pairs("path-data", "data"), 
)