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

[davidebbo]

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.

[lindydonna]

Turns out I created a duplicate item. Here are a few more details:

There are two parts:

1. When calling the admin API, return an invocation ID
2. Provide an API to get the logs for that invocation ID

Open question: for HTTP triggers, the portal just calls the endpoint directly and does not use the admin API. Need to come up with a design to handle this.

[lindydonna]

Looks like we have another duplicate: #1122