Update defguard_core > tests > integration > grpc to match reversed communication

[kchudy]

Update the existing mock gateway testing setup to use the new reversed communication approach:

• update test server to setup gateway handlers for each mock gateway
• use CoreRequest/CoreResponse to wrap messages to fit new protobuf definitions
• remove JWT auth which is no longer necessary
• run the test server without the certificate setup
• verify that only valid peer stat updates are sent to the session manager (ones that include peer endpoint)

NOTES:

• the goal of those tests is to verify that our gateway handler logic works as expected and not the whole core <-> gw SSL communication setup, reconnects etc
• session-related logic is now moved to the session manager which removes some tests
• the logic about when a gateway is "connected" is also significantly different now

Result: Existing tests in the specified module are reenabled and pass in CI. Test which have become irrelevant are removed.