Set the user's identity as the confidentiality tag in invocation channels for HTTP and gRPC server nodes

[rbehjati]

Each incoming gRPC/HTTP invocation is represented by a message containing two Channel handles: a "request receiver" and a "response sender".

Currently the "response sender" gets the public_untrusted label. However, as explained in the docs, it should have a confidentiality component that is set based on the identity of the user. This is needed to avoid any accidental data leaks.

[tiziano88]

@rbehjati I believe your changes already fix this, so assigning to you, but let me know if that's not the case.

[rbehjati]

Yes. But partially. I'll update the description here, and can create a separate issue to track setting the user identity as the integrity label. Do we have a top-level issue for integrity labels?

[rbehjati]

Closing this now, but the work on identity-related integrity labels is going to be part of IFC v1, and #1715.