platform B eventing integration

[lkingland]

The adapter implementation for platform B is currently a stub. It will need to be fleshed out to include the implementation expected of a platform B Worker, which in particular means pull-based event handling with an internal work queue.

The grid service will request new messages from the host, and will treat the running function instance (expected to be running on a well-known local port) as a synchronous handler for each request. This necessitates that there also be a few configuration settings (which can be hard-coded constants for now): for how many concurrent goroutines to create to handle inbound events, and another for how many events to pull from the host into local memory as the local work queue. A subsequent ticket will address plumbing these through from main and the detection of platform B as the runtime environment.

In addition, the inbound and outbound events may need to be converted to/from CloudEvents before invoking the local function. This will hopefully be only temporary scaffolding code as the platform B project may end up running CloudEvents in the long run.