Test PBJ GRPC keep-alive machinery

GRPC/HTTP2 rely on keep-alive pings to keep long-running connections alive. We've experienced issues with closed connections on the Mirror Node side, but that was (at least partially) caused by disabling the keep-alive pings on the MN side.

Regardless, we want to write a test that would verify if a client sending proper keep-alive pings is able to maintain a long-running connection to a server.

Start by locating the PBJ GRPC client and server keep-alive test entry points, then review how existing GRPC/HTTP2 connection tests are run. Add coverage showing that a client sending proper keep-alive pings maintains a long-running connection; done means the connection remains usable without premature closure.

Written by the indexing model from the issue text.