Add policy URI to each endpoint token's SecurityPolicyUri

[haotruongnhat]

The opc-ua client backend open62541 scans for endpoint tokens' security URI on connection. This PR to resolve the case that open62541 to skip token which is missing that URI.

[cziebuhr]

According to https://reference.opcfoundation.org/Core/Part4/v105/docs/7.42, the security policy for the SecureChannel is used if this value is null or empty, so this should not have been an issue here. But I will report other findings in that area in the next days.

[haotruongnhat]

@cziebuhr The UAExpert software uses the official OPC-UA backend, which does follow the document stated above. Therefore, the connection works fine with this tool.

However, in the open-sourced open62541 backend, it checks only the securityPolicyUri in each available token, and skips the token if it is null or empty. It's quite tricky.