Make function execution API return an execution ID that can be used to get logs

Today, when Portal executes a method, the only way it has to get the logs is to rely on the log stream, which doesn't work well.

Instead, we should switch to a model where we get back an invocation id (e.g. in a header) that can be used to easily get all the relevant logs (and counters) for this one invocation.

Start by tracing the Portal function-execution request and the existing log-stream path; the issue names no files or tests. Determine where an invocation ID could be returned and how logs and counters are retrieved for that invocation. Done means the API returns an ID and retrieves the relevant logs and counters without relying on the stream.

Written by the indexing model from the issue text.