Expose a new invoke function via gRPC from Worker Executor for incoming HTTP requests

[vigoo]

This is the first step in supporting workers implementing the wasi-http incoming-handler interface (https://wa.dev/wasi:http#incoming-handler).

The primary challenge is that this interface takes an incoming-request handle to a host resource, and returns with a outgoing-response handle to a host resource, which is not something we can directly invoke using Golem's existing invokation API.

Instead, as part of this ticket, we define:

• A gRPC definition of HTTP request->response handler, using gRPC streaming for the request and response body
• A special invocation mode in worker executor, wrapping the existing one (on the level of invocation.rs) that does the following:
  • Take the request information (coming via gRPC) and set up a host incoming-resource instance
  • Invoke the worker's exported wasi:http/incoming-handler:{handle} function with this resource
  • Take the returned outgoing-response resource and convert it back to the protobuf response structure
• This new invocation mode needs to be implemented on all layers (Worker, including the invocation loop) and wired to the gRPC handler
• To make the scope of the task smaller, it is enough for this ticket to NOT do actual request/response body streaming
• Likely requires changes to the oplog too (to be decided by the implementor), some options:
  • Maybe it is possible to not introduce new invocation start/end entries, but instead use the existing ones, converting the request/response data to Value. Or maybe it is easier to have dedicated new ones. Think of the current ExportedFunctionInvoked, ExportedFunctionCompleted and PendingWorkerInvocation entries.
• Support this new call in the test DSL
• Write worker executor test(s) proving this works

With this ticket done we can call worker executor using a representation of a "http request" and get back a representation of a "http response", while the worker itself exports only the wasi-http incoming handler interface.

Follow-up tickets add streaming support and wires this functionality into the worker bridge.